drm/i915: don't set modes for 2 connectors on the same encoder

In some cases we have more than 1 connector associated to an encoder
(e.g., SDVO, Haswell DP/HDMI) and we can only set a mode for one of
these connectors. If we only allowed modesets for connected connectors
we would never need this patch, but since we do allow modeset for
disconnected connectors we may see user space trying to set modes on
the two connectors attached to the same encoder, so we need to forbid
that.

This problem can be reproduced by running the following
intel-gpu-tools test case:
  ./kms_setmode --run-subtest clone-exclusive-crtc

Thanks to Daniel Vetter for providing a version of this patch on
pastebin.

Credits-to: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -9923,17 +9923,21 @@ intel_modeset_stage_output_state(struct drm_device *dev,
 	/* Check for any encoders that needs to be disabled. */
 	list_for_each_entry(encoder, &dev->mode_config.encoder_list,
 			    base.head) {
+		int num_connectors = 0;
 		list_for_each_entry(connector,
 				    &dev->mode_config.connector_list,
 				    base.head) {
 			if (connector->new_encoder == encoder) {
 				WARN_ON(!connector->new_encoder->new_crtc);
-
-				goto next_encoder;
+				num_connectors++;
 			}
 		}
-		encoder->new_crtc = NULL;
-next_encoder:
+
+		if (num_connectors == 0)
+			encoder->new_crtc = NULL;
+		else if (num_connectors > 1)
+			return -EINVAL;
+
 		/* Only now check for crtc changes so we don't miss encoders
 		 * that will be disabled. */
 		if (&encoder->new_crtc->base != encoder->base.crtc) {