drm/i915: Split GEN6 PPGTT cleanup

This cleanup is similar to the GEN8 cleanup (though less necessary).
Having everything split will make cleaning the initialization path error
paths easier to understand.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>