drm/i915: Extract intel_panel_mode_valid()

Extract intel_panel_mode_valid() from the eDP code to a generic helper.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210923200109.4459-2-ville.syrjala@linux.intel.com


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