drm/radeon: switch UVD code to use UVD_NO_OP for padding

Replace packet2's with packet0 writes to UVD_NO_OP.  The
value written to UVD_NO_OP does not matter.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>