video: screen_info: Add pixel-format helper for linear framebuffers

Add screen_info_pixel_format(), which converts a screen_info's
information about the color format to struct pixel_format. The encoding
within the screen_info structure is complex and therefore prone to
errors. Later patches will convert callers to use the pixel format.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250714151513.309475-3-tzimmermann@suse.de