drm/tinydrm: Add tinydrm_xrgb8888_to_gray8() helper

Drm has no monochrome or greyscale support so add a conversion
from the common format XR24.

Also reorder includes into the common order.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1496934875-51984-4-git-send-email-noralf@tronnes.org