drm/fourcc: Add drm_format_info.is_color_indexed flag

Add a flag to struct drm_format_info to indicate if a format is
color-indexed, similar to the existing .is_yuv flag.

This way generic code and drivers can just check this flag, instead of
checking against a list of fourcc formats.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/90cd390b2b4d481661f966de7b504f1702d80dfd.1657294931.git.geert@linux-m68k.org