We'll add the GBM format code as the native visual ID for EGLConfigs
when running on GBM.  This patch lets the drm backend pass in the
format code it's using with KMS and make sure we get a confing that
matches.  In the future, mesa will add support for 10 bpc configs
which will match the "at least 8 color bits" requirement.  By also
matching the native visual ID, we avoid rendering XRGB2101010 into a
XRGB8888 KMS framebuffer.