firmware: sysfb: Move bpp-depth calculation into screen_info helper

Move the calculation of the bits per pixels for screen_info into a
helper function. This will make it available to other callers besides
the firmware code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250401094056.32904-14-tzimmermann@suse.de