drm/format-helper: Rework XRGB8888-to-GRAY8 conversion

drm/format-helper: Rework XRGB8888-to-GRAY8 conversion

Update XRGB8888-to-GRAY8 conversion to support struct iosys_map
and convert all users. Although these are single-plane color formats,
the new interface supports multi-plane formats for consistency with
drm_fb_blit().

v2:
	* update documentation (Sam)
	* add TODO on vaddr location (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220808125406.20752-12-tzimmermann@suse.de