drm/format-helper: introduce drm_fb_xrgb8888_to_gray2()

Convert XRGB8888 to 2bit grayscale.

It uses drm_fb_xrgb8888_to_gray8() to convert the pixels to gray8 as an
intermediate step before converting to gray2.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250721-st7571-format-v2-5-159f4134098c@gmail.com


Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>