drm/cirrus: Replace cpp value with format

Using components per pixel to describe a color format is obsolete.
Use the format info and 4CC value instead.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-3-tzimmermann@suse.de