drm: Make passing of format info to drm_helper_mode_fill_fb_struct() mandatory

Now that everyone passes along the format info to
drm_helper_mode_fill_fb_struct() we can make this behaviour
mandatory and drop the extra lookup.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250701090722.13645-20-ville.syrjala@linux.intel.com