drm/st7571-i2c: add support for 2bit grayscale for XRGB8888

Add support for 2bit grayscale and use it for XRGB8888 when grayscale is
supported.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Link: https://lore.kernel.org/r/20250721-st7571-format-v2-6-159f4134098c@gmail.com


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