drm/ofdrm: 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. Keeping the existing switch statement just duplicates the
driver's existing logic for format detection.

Also remove the FIXME comment from the call to drm_fbdev_generic_setup()
as the driver now handles color depth and bpp values correctly.

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/20221123115348.2521-5-tzimmermann@suse.de