drm/i915: Fixup global gtt cleanup

The global gtt is setup up in 2 parts, so we need to be careful
with the cleanup. For consistency shovel it all into the ->cleanup
callback, like with ppgtt.

Noticed because it blew up in the out_gtt: cleanup code while
fiddling with the vgacon code.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>