video: pixel_format: Add compare helpers

Add helpers that compare two pixel-format descriptions against
each other.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250714151513.309475-2-tzimmermann@suse.de