drm/edid: split drm_add_edid_modes() to two

Reduce the size of the function that actually modifies the EDID.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/437c3c79f68d1144444fb2dd18a678f3aa97272c.1648477901.git.jani.nikula@intel.com