drm/i915: Add intel_panel_highest_mode()

Add a function to get the fixed_mode with the highest clock.
The plan is to use this for the link bw calculation on seamless
DRRS panels so that we alwasy end up with the same link params
regardless of the requested refresh rate. This will allow fastset
to do seamless refresh rate changes based on userspace request
instead of having to go for a full modeset.

TODO: the function name isn't great

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220907091057.11572-15-ville.syrjala@linux.intel.com