gallium: Add buffer and configs handling or fp16 formats

gallium: Add buffer and configs handling or fp16 formats

Expose configs when allow_fp16_configs has been enabled and
DRI_LOADER_CAP_FP16 is set in the loader.

Also, make kms_swrast_dri respect format bpp, to allow for allocating
buffers wider than 32 bpp.

Make fp16 opt-in for gallium.

Signed-off-by: Kevin Strasser <kevin.strasser@intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>