drm/fb-helper: Replace bpp/depth parameter by color mode

Replace the combination of bpp and depth with a single color-mode
argument. Handle special cases in simpledrm and ofdrm. Hard-code
XRGB8888 as fallback format for cases where no given format works.

The color-mode argument accepts the same values as the kernel's video
parameter. These are mostly bpp values between 1 and 32. The exceptions
are 15, which has a color depth of 15 and a bpp value of 16; and 32,
which has a color depth of 24 and a bpp value of 32.

v4:
	* add back lost test for bpp_specified (Maira)
	* add Fixes tag (Daniel)
v3:
	* fix ofdrm build (Maxime)
v2:
	* minimize changes (Daniel)
	* use drm_driver_legacy_fb_format() (Daniel)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Maíra Canal <mcanal@igalia.com> # vc4 and vkms
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: 37c90d58 ("drm/fb-helper: Fix single-probe color-format selection")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230106112324.22055-1-tzimmermann@suse.de