drm: Add drm_connector_has_possible_encoder()

Add a small helper for checking whether a connector and
encoder are associated with each other.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180628131315.14156-8-ville.syrjala@linux.intel.com


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>