drm/i915: Use drm_atomic_get_existing_plane_state

We want to encapsulate the drm_atomic_state internals.

v2: Use intel_atomic_get_existing_plane_state (Maarten).
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464818821-5736-3-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/i915/intel_atomic.c

@@ -223,7 +223,8 @@ int intel_atomic_setup_scalers(struct drm_device *dev,
 				continue;
 			}
 
-			plane_state = to_intel_plane_state(drm_state->plane_states[i]);
+			plane_state = intel_atomic_get_existing_plane_state(drm_state,
+									    intel_plane);
 			scaler_id = &plane_state->scaler_id;
 		}