drm/i915: fixup per-crtc locking in intel_release_load_detect_pipe

One of the early return cases missed the mutex unlocking. Hilarity
ensued.

This regression has been introduced in

commit 7b24056be6db7ce907baffdd4cf142ab774ea60c
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Dec 12 00:35:33 2012 +0100

    drm: don't hold crtc mutexes for connector ->detect callbacks

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=59750
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Cancan Feng <cancan.feng@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/i915/intel_display.c

@@ -6513,6 +6513,7 @@ void intel_release_load_detect_pipe(struct drm_connector *connector,
 			drm_framebuffer_unreference(old->release_fb);
 		}
 
+		mutex_unlock(&crtc->mutex);
 		return;
 	}