lavu/hwcontext_vaapi: Skip 'vgem' driver

There can be more than one available render node, and it's not
guaranteed the first node we come across is the correct one. In
particular, 'vgem' devices are common, and are
never VAAPI-enabled and thus not valid here.

We have a 'kernel_driver' arg already for specifying a single driver we
*do* want, but it doesn't support a negation, nor a list. It's easier
just to automatically skip 'vgem' anyway, to avoid foisting this burden
on users.

This has precedent in libva-utils already:

  bfb6b98ed62a exclude vgem node and invalid drm node in vainfo
  bfb6b98ed6

Signed-off-by: Brian Norris <briannorris@chromium.org>

libavutil/hwcontext_vaapi.c

@@ -1700,6 +1700,7 @@ static int vaapi_device_create(AVHWDeviceContext *ctx, const char *device,
             char path[64];
             int n, max_devices = 8;
 #if CONFIG_LIBDRM
+            drmVersion *info;
             const AVDictionaryEntry *kernel_driver;
             kernel_driver = av_dict_get(opts, "kernel_driver", NULL, 0);
 #endif
@@ -1713,9 +1714,15 @@ static int vaapi_device_create(AVHWDeviceContext *ctx, const char *device,
                     break;
                 }
 #if CONFIG_LIBDRM
+                info = drmGetVersion(priv->drm_fd);
+                if (!info) {
+                    av_log(ctx, AV_LOG_VERBOSE,
+                           "Failed to get DRM version for device %d.\n", n);
+                    close(priv->drm_fd);
+                    priv->drm_fd = -1;
+                    continue;
+                }
                 if (kernel_driver) {
-                    drmVersion *info;
-                    info = drmGetVersion(priv->drm_fd);
                     if (strcmp(kernel_driver->value, info->name)) {
                         av_log(ctx, AV_LOG_VERBOSE, "Ignoring device %d "
                                "with non-matching kernel driver (%s).\n",
@@ -1730,12 +1737,20 @@ static int vaapi_device_create(AVHWDeviceContext *ctx, const char *device,
                            "with matching kernel driver (%s).\n",
                            n, info->name);
                     drmFreeVersion(info);
-                } else
-#endif
-                {
-                    av_log(ctx, AV_LOG_VERBOSE, "Trying to use "
-                           "DRM render node for device %d.\n", n);
+                    break;
+                // drmGetVersion() ensures |info->name| is 0-terminated.
+                } else if (!strcmp(info->name, "vgem")) {
+                    av_log(ctx, AV_LOG_VERBOSE,
+                           "Skipping vgem node for device %d.\n", n);
+                    drmFreeVersion(info);
+                    close(priv->drm_fd);
+                    priv->drm_fd = -1;
+                    continue;
                 }
+                drmFreeVersion(info);
+#endif
+                av_log(ctx, AV_LOG_VERBOSE, "Trying to use "
+                       "DRM render node for device %d.\n", n);
                 break;
             }
             if (n >= max_devices)