drm/simpledrm: Support the XB24/AB24 format

Add XB24 and AB24 to the list of supported formats. The format helpers
support conversion to these formats and they are documented in the
simple-framebuffer device tree bindings.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230120173103.4002342-8-thierry.reding@gmail.com