drm/i915: Introduce intel_panel_get_modes()

Several connectors want to return the fixed_mode from .get_modes(),
add a helper to do that (and hide the details inside intel_panel.c).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220311172428.14685-8-ville.syrjala@linux.intel.com


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