Merge pull request #17028 from xipingyan:patch-1

* Update OpenCVFindVA_INTEL.cmake

When set env VA_INTEL_IOCL_ROOT, "if($ENV{VA_INTEL_IOCL_ROOT})" don't work.
    My modification as follow.
    
    -    if($ENV{VA_INTEL_IOCL_ROOT})
    +    if(DEFINED ENV{VA_INTEL_IOCL_ROOT})
    
    Refer: https://cmake.org/cmake/help/latest/variable/ENV.html

* based on merge comment, update code

cmake/OpenCVFindVA_INTEL.cmake

@@ -5,9 +5,8 @@
 # VA_INTEL_IOCL_ROOT - root of Intel OCL installation
 
 if(UNIX AND NOT ANDROID)
-    if($ENV{VA_INTEL_IOCL_ROOT})
-        set(VA_INTEL_IOCL_ROOT $ENV{VA_INTEL_IOCL_ROOT})
-    else()
+    ocv_check_environment_variables(VA_INTEL_IOCL_ROOT)
+    if(NOT DEFINED VA_INTEL_IOCL_ROOT)
       set(VA_INTEL_IOCL_ROOT "/opt/intel/opencl")
     endif()