drm/sysfb: Provide single mode-init helper

Merge the mode-init functions of ofdrm and simpledrm to the new helper
drm_sysfb_mode(). Also implement the DPI defaults there. Replace the
code in each driver with the shared helper.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250401094056.32904-7-tzimmermann@suse.de