drm/atomic: Add encoder_mask to crtc_state, v3.

This allows iteration over encoders without requiring connection_mutex.

Changes since v1:
- Add a set_best_encoder helper function and update encoder_mask inside
  it.
Changes since v2:
- Relax the WARN_ON(!crtc), with explanation.
- Call set_best_encoder when connector is moved between crtc's.
- Add some paranoia to steal_encoder to prevent accidentally setting
  best_encoder to NULL.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/56AA200A.6070501@linux.intel.com
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 0ea5a4e..e1e7cde 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -15591,6 +15591,7 @@
 		crtc->base.state->active = crtc->active;
 		crtc->base.enabled = crtc->active;
 		crtc->base.state->connector_mask = 0;
+		crtc->base.state->encoder_mask = 0;
 
 		/* Because we only establish the connector -> encoder ->
 		 * crtc links if something is active, this means the
@@ -15830,6 +15831,8 @@
 				 */
 				encoder->base.crtc->state->connector_mask |=
 					1 << drm_connector_index(&connector->base);
+				encoder->base.crtc->state->encoder_mask |=
+					1 << drm_encoder_index(&encoder->base);
 			}
 
 		} else {