cmake: allow to specify own libva paths

via CMake:
- `-DVA_LIBRARIES=/opt/intel/mediasdk/lib64/libva.so.2\;/opt/intel/mediasdk/lib64/libva-drm.so.2`

cmake/OpenCVFindVA.cmake

@@ -12,7 +12,9 @@ endif()
 
 if(VA_INCLUDE_DIR)
     set(HAVE_VA TRUE)
-    set(VA_LIBRARIES "-lva" "-lva-drm")
+    if(NOT DEFINED VA_LIBRARIES)
+      set(VA_LIBRARIES "va" "va-drm")
+    endif()
 else()
     set(HAVE_VA FALSE)
     message(WARNING "libva installation is not found.")