lavc/aarch64: Fix ff_pred8x8_plane_neon_10

Fix test failure on aarch64:
./tests/checkasm/checkasm --test=h264pred 479612

The mismatch between neon and C functions can also be reproduced using the following bitstream and command line.

wget https://streams.videolan.org/ffmpeg/incoming/intra8x8pred_10bit.264
 ./ffmpeg -cpuflags 0  -threads 1 -i intra8x8pred_10bit.264  -f framemd5 -y md5_ref
 ./ffmpeg              -threads 1 -i intra8x8pred_10bit.264  -f framemd5 -y md5_neon

Signed-off-by: Bin Peng <pengbin@visionular.com>
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit decc9e643c)

libavcodec/aarch64/h264pred_neon.S

@@ -595,12 +595,11 @@ function ff_pred8x8_plane_neon_10, export=1
         ssubl           v2.4s,  v2.4h,  v3.4h
         ext             v0.16b, v0.16b, v0.16b, #14
         mov             v0.h[0],  wzr
-        mul             v0.8h,  v0.8h,  v5.h[0]
         dup             v1.4s,  v2.s[0]
         dup             v2.4s,  v2.s[0]
         dup             v3.8h,  v5.h[1]
-        saddw           v1.4s,  v1.4s,  v0.4h
-        saddw2          v2.4s,  v2.4s,  v0.8h
+        smlal           v1.4s,  v0.4h,  v5.h[0]
+        smlal2          v2.4s,  v0.8h,  v5.h[0]
         mov             w3,  #8
         mvni            v4.8h,  #0xFC,  lsl #8 // 1023 for clipping
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