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

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

[ Upstream commit eb1f4adf9101573fc2347978a60d71c4f1176cca ]

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
Stable-dep-of: 6b481ab0e685 ("drm/nouveau: select FW caching")
Signed-off-by: Sasha Levin <sashal@kernel.org>