drm/repaper: Use format helper for xrgb8888 to monochrome conversion

There is now a drm_fb_xrgb8888_to_mono_reversed() helper function to do
format conversion from XRGB8888 to reversed monochrome.

Use that helper and remove the open coded version in the repaper driver.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223193735.213185-1-javierm@redhat.com