drm/i915: Cleanup aliasging ppgtt alongside the global gtt

Also remove related WARN_ONs which seem to have been hit since a rather
long time. But apperently no one noticed since our module reload is
already WARNING-infested :(

Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>