drm/amdgpu: Remove default best_encoder hook from DC

For atomic driver this is the default, no need to reimplement it. We
still need to keep the copypasta for not-atomic drivers though, since
no one polished the legacy crtc helpers as much as the atomic ones.

v2: amdgpu uses ->best_encoder internally, give it a local copy. It
might be a good idea to merge the connector and encoder into one
amdgpu_dm_sink structure, that might match DC internals better. At
least for non-DPMST outputs. Kudos to Ville for spotting this.

v3: Rebase onto a487411 ("drm/amd/display: Use DRM helper for
best_encoder").

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Cc: Tony Cheng <Tony.Cheng@amd.com>
Cc: "Leo (Sunpeng) Li" <sunpeng.li@amd.com>
Cc: Shirish S <shirish.s@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-2-daniel.vetter@ffwll.ch

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -3070,7 +3070,6 @@ amdgpu_dm_connector_helper_funcs = {
 	 */
 	.get_modes = get_modes,
 	.mode_valid = amdgpu_dm_connector_mode_valid,
-	.best_encoder = drm_atomic_helper_best_encoder
 };
 
 static void dm_crtc_helper_disable(struct drm_crtc *crtc)
@@ -3473,14 +3472,17 @@ static int to_drm_connector_type(enum signal_type st)
 	}
 }
 
+static struct drm_encoder *amdgpu_dm_connector_to_encoder(struct drm_connector *connector)
+{
+	return drm_encoder_find(connector->dev, NULL, connector->encoder_ids[0]);
+}
+
 static void amdgpu_dm_get_native_mode(struct drm_connector *connector)
 {
-	const struct drm_connector_helper_funcs *helper =
-		connector->helper_private;
 	struct drm_encoder *encoder;
 	struct amdgpu_encoder *amdgpu_encoder;
 
-	encoder = helper->best_encoder(connector);
+	encoder = amdgpu_dm_connector_to_encoder(connector);
 
 	if (encoder == NULL)
 		return;
@@ -3607,14 +3609,12 @@ static void amdgpu_dm_connector_ddc_get_modes(struct drm_connector *connector,
 
 static int amdgpu_dm_connector_get_modes(struct drm_connector *connector)
 {
-	const struct drm_connector_helper_funcs *helper =
-			connector->helper_private;
 	struct amdgpu_dm_connector *amdgpu_dm_connector =
 			to_amdgpu_dm_connector(connector);
 	struct drm_encoder *encoder;
 	struct edid *edid = amdgpu_dm_connector->edid;
 
-	encoder = helper->best_encoder(connector);
+	encoder = amdgpu_dm_connector_to_encoder(connector);
 
 	if (!edid || !drm_edid_is_valid(edid)) {
 		amdgpu_dm_connector->num_modes =