drm/i915: Clean up EDID downclock mode lookup

Rename intel_find_panel_downclock() to intel_panel_edid_downclock_mode()
to make it clear it's looking for the downclock mode in the EDID.
And while at it polish the implementation a bit as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321132446.22394-6-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jani Nikula <jani.nikula@intel.com>