drm/edid: Extract drm_gtf2_mode()

Extract the GTF vs. GTF2 logic into a separate function.
We'll have a second user soon.

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/20220927170006.27855-5-ville.syrjala@linux.intel.com