drm/i810: don't acces hw regs in lastclose

i810 uses a userspace provided mmio map using the drm core map
infrastructure. By the time we reach lastclose, this is all gone
and our mmio_map pointer points at freed memory. Depending upon
luck that still works, most often it just oopses.

Aside: drm maps aren't refcounted, so userspace can essentially oops
the kernel any time it wants to. Who cares.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>