drm: Fix comment referring to the long gone ->probe() connector vfunc

drm_helper_probe_single_connector_modes() can be used to implement
->fill_modes(), not ->probe().

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>