drm/format-helper: Add XRGB8888-to-RGB332 to drm_fb_blit()

List the conversion from XRGB8888-to-RGB332 in drm_fb_blit(), so that
drivers based on sysfb-helpers can use it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250714151513.309475-7-tzimmermann@suse.de