drm/i915/panel: add panel register/unregister

Add panel register/unregister functions, and handle backlight
register/unregister from there. This is in preparation for adding more
panel specific register/unregister functionality.

Cc: Lee Shawn C <shawn.c.lee@intel.com>
Tested-by: Lee Shawn C <shawn.c.lee@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@gmail.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/b737d4bc8b91df630cd4db4648f3a3571989cfd8.1749199013.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>