drm/atomic-helper: Check encoder/crtc constraints

This was totally lost when I originally created the atomic helpers.

We probably should also check possible_clones in the helpers, but
since the legacy ones didn't do that this is for a separate patch.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Stone <daniels@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447868808-10266-1-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_atomic_helper.c

@@ -210,6 +210,14 @@ update_connector_routing(struct drm_atomic_state *state, int conn_idx)
 		return -EINVAL;
 	}
 
+	if (!drm_encoder_crtc_ok(new_encoder, connector_state->crtc)) {
+		DRM_DEBUG_ATOMIC("[ENCODER:%d:%s] incompatible with [CRTC:%d]\n",
+				 new_encoder->base.id,
+				 new_encoder->name,
+				 connector_state->crtc->base.id);
+		return -EINVAL;
+	}
+
 	if (new_encoder == connector_state->best_encoder) {
 		DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] keeps [ENCODER:%d:%s], now on [CRTC:%d]\n",
 				 connector->base.id,