drm/format-helper: Support the AB24/XB24 formats

Add a conversion helper for the AB24 and XB24 formats to use in
drm_fb_blit().

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230120173103.4002342-7-thierry.reding@gmail.com