drm/fbdev-helper: Move color-mode lookup into 4CC format helper

The color mode as specified on the kernel command line gives the user's
preferred color depth and number of bits per pixel. Move the
color-mode-to-format conversion from fbdev helpers into a 4CC helper,
so that it can be shared among DRM clients.

v2:
- fix grammar in commit message (Laurent)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240924071734.98201-2-tzimmermann@suse.de