drm/i915: Introduce intel_panel_preferred_fixed_mode()

There are a couple of cases where we essentially just want to
get/check the preferred fixed mode of the panel. Add a small
helper for that to abstract away the direct pointer lookup.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220311172428.14685-9-ville.syrjala@linux.intel.com


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