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>