drm/simpledrm: Set preferred depth from format of scanout buffer

Set the preferred depth from the format of the scanout buffer. The
value cannot be hardcoded, as the scanout buffer is only known at
runtime. Also derive the fbdev emulation's bpp value from the scanout
format.

v2:
	* fix commit-message typo

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20221123115348.2521-6-tzimmermann@suse.de