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>

src/gallium/frontends/xvmc/subpicture.c

@@ -114,6 +114,7 @@ static int PipeToComponentOrder(struct pipe_screen *screen,
 
    switch (format) {
    case PIPE_FORMAT_B8G8R8X8_UNORM:
+      *palette_format = PIPE_FORMAT_NONE;
       return 0;
 
    case PIPE_FORMAT_A4R4_UNORM:
@@ -136,6 +137,7 @@ static int PipeToComponentOrder(struct pipe_screen *screen,
 
    default:
       XVMC_MSG(XVMC_ERR, "[XvMC] Unrecognized PIPE_FORMAT 0x%08X.\n", format);
+      *palette_format = PIPE_FORMAT_NONE;
       component_order[0] = 0;
       component_order[1] = 0;
       component_order[2] = 0;