xvmc: Initialize pallete_format in PipeToComponentOrder

Fixes a warning in GCC 9+ that's been bugging me for a very long time when building Mesa.

Signed-off-by: Joshua Ashton <joshua@froggi.es>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
61 jobs for !12956 with xvmc-palltete-warning-fix in 10 seconds (queued for 16 seconds)
latest merge request