drm/i915: clear opregon->lid_state after we unmap it

We don't seem to be using the pointer after it's unmapped, so this
patch doesn't fix any bug I can reproduce.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_opregion.c

@@ -657,6 +657,7 @@ void intel_opregion_fini(struct drm_device *dev)
 	opregion->swsci = NULL;
 	opregion->asle = NULL;
 	opregion->vbt = NULL;
+	opregion->lid_state = NULL;
 }
 
 static void swsci_setup(struct drm_device *dev)