i915: fix lock imbalance on error path...

While investigating Intel i5 Arrandale GPU lockups with -rc4, I
noticed a lock imbalance.
Signed-off-by: NDaniel J Blueman <daniel.blueman@gmail.com>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_drv.c

@@ -370,6 +370,7 @@ int i965_reset(struct drm_device *dev, u8 flags)
 		}
 	} else {
 		DRM_ERROR("Error occurred. Don't know how to reset this chip.\n");
+		mutex_unlock(&dev->struct_mutex);
 		return -ENODEV;
 	}