drm/panel: abstract of_panel_find()

Add a helper to wrap OF-specific calls in drm_panel_add_follower() in
preparation for adding an ACPI equivalent in the future. No functional
changes.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250522100036.2529624-1-jani.nikula@intel.com


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