st/va: clear pointers for mpeg2 quantiser matrices

This is to fix VA-API issues with GStreamer and MPEG2.
Since gstreamer does not pass quantiser matrices with each frame, invalid
pointers were being passed to the driver. This patch addresses the same.

Signed-off-by: Indrajit Das <indrajit-kumar.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
(cherry picked from commit 338638a8)