This header is for sharing fallback definitions for drm_fourcc.h. A new
test in tests/yuv-buffer-test.c is going to be needing XYUV8888 format,
and more new formats will be expected with HDR supports.

Share these fallback definitions in one place instead of copying them
all over.

All users of drm_fourcc.h are converted to include weston-drm-fourcc.h
instead for consistency: have the same definitions available everywhere.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>