drm/atomic-helper: Add an atomice best_encoder callback

With legacy helpers all the routing was already set up when calling
best_encoder and so could be inspected. But with atomic it's staged,
hence we need a new atomic compliant callback for drivers which need
to inspect the requested state and can't just decided the best encoder

This is needed to fix up i915 dp mst where we need to pick the right
encoder depending upon the requested CRTC for the connector.

v2: Don't forget to amend the kerneldoc

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Acked-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index aac2122..8694ca9 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -196,7 +196,12 @@
 	funcs = connector->helper_private;
-	new_encoder = funcs->best_encoder(connector);
+	if (funcs->atomic_best_encoder)
+		new_encoder = funcs->atomic_best_encoder(connector,
+							 connector_state);
+	else
+		new_encoder = funcs->best_encoder(connector);
 	if (!new_encoder) {
 		DRM_DEBUG_ATOMIC("No suitable encoder found for [CONNECTOR:%d:%s]\n",
diff --git a/include/drm/drm_crtc_helper.h b/include/drm/drm_crtc_helper.h
index c8fc187..918aa68 100644
--- a/include/drm/drm_crtc_helper.h
+++ b/include/drm/drm_crtc_helper.h
@@ -168,6 +168,7 @@
  * @get_modes: get mode list for this connector
  * @mode_valid: is this mode valid on the given connector? (optional)
  * @best_encoder: return the preferred encoder for this connector
+ * @atomic_best_encoder: atomic version of @best_encoder
  * The helper operations are called by the mid-layer CRTC helper.
@@ -176,6 +177,8 @@
 	enum drm_mode_status (*mode_valid)(struct drm_connector *connector,
 					   struct drm_display_mode *mode);
 	struct drm_encoder *(*best_encoder)(struct drm_connector *connector);
+	struct drm_encoder *(*atomic_best_encoder)(struct drm_connector *connector,
+						   struct drm_connector_state *connector_state);
 extern void drm_helper_disable_unused_functions(struct drm_device *dev);