st/mesa: Fall back on R3G3B2 for R3_G3_B2

It's simpler for Panfrost to use R3G3B2 instead of B2G3R3, but
format_map only listed the BGR variation.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4292>
1 file changed