drm/i915: check connector->encoder before using it.

DP MST will need connectors that aren't connected to specific
encoders, add some checks in advance to avoid oopses.

Reviewed-by: Todd Previte <tprevite@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 026f0a3..12d77b5 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -1165,6 +1165,8 @@
 	dev_priv->hpd_event_bits = 0;
 	list_for_each_entry(connector, &mode_config->connector_list, head) {
 		intel_connector = to_intel_connector(connector);
+		if (!intel_connector->encoder)
+			continue;
 		intel_encoder = intel_connector->encoder;
 		if (intel_encoder->hpd_pin > HPD_NONE &&
 		    dev_priv->hpd_stats[intel_encoder->hpd_pin].hpd_mark == HPD_MARK_DISABLED &&
@@ -1195,6 +1197,8 @@
 
 	list_for_each_entry(connector, &mode_config->connector_list, head) {
 		intel_connector = to_intel_connector(connector);
+		if (!intel_connector->encoder)
+			continue;
 		intel_encoder = intel_connector->encoder;
 		if (hpd_event_bits & (1 << intel_encoder->hpd_pin)) {
 			if (intel_encoder->hot_plug)