drm/connector: Share with non-atomic drivers the function to get the single encoder

This 3 non-atomic drivers all have the same function getting the
only encoder available in the connector, also atomic drivers have
this fallback. So moving it a common place and sharing between atomic
and non-atomic drivers.

While at it I also removed the mention of
drm_atomic_helper_best_encoder() that was renamed in
commit 297e30b5d9 ("drm/atomic-helper: Unexport
drm_atomic_helper_best_encoder").

v3: moving drm_connector_get_single_encoder to drm_kms_helper module

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: dri-devel@lists.freedesktop.org
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190913232857.389834-1-jose.souza@intel.com

drivers/gpu/drm/udl/udl_connector.c

@@ -90,13 +90,6 @@ udl_detect(struct drm_connector *connector, bool force)
 	return connector_status_connected;
 }
 
-static struct drm_encoder*
-udl_best_single_encoder(struct drm_connector *connector)
-{
-	int enc_id = connector->encoder_ids[0];
-	return drm_encoder_find(connector->dev, NULL, enc_id);
-}
-
 static int udl_connector_set_property(struct drm_connector *connector,
 				      struct drm_property *property,
 				      uint64_t val)
@@ -120,7 +113,6 @@ static void udl_connector_destroy(struct drm_connector *connector)
 static const struct drm_connector_helper_funcs udl_connector_helper_funcs = {
 	.get_modes = udl_get_modes,
 	.mode_valid = udl_mode_valid,
-	.best_encoder = udl_best_single_encoder,
 };
 
 static const struct drm_connector_funcs udl_connector_funcs = {