drm/panel: use fwnode based lookups for panel followers

Use firmware node based lookups for panel followers, to make the code
independent of OF and device tree, and make it work also for ACPI with
an appropriate _DSD.

ASL example:

	Package (0x02)
	{
		"panel", \_SB.PCI0.GFX0.LCD0
	}

v2:
- Update comments (Doug, Arun)
- s/IS_ERR_OR_NULL/IS_ERR/ (Doug)

Suggested-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Neil Armstrong <neil.armstrong@linaro.org>
Cc: Jessica Zhang <jessica.zhang@oss.qualcomm.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Doug Anderson <dianders@chromium.org>
Cc: Lee Shawn C <shawn.c.lee@intel.com>
Tested-by: Lee Shawn C <shawn.c.lee@intel.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250610094132.3240567-1-jani.nikula@intel.com


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