drm/i915/bdw: Split up PPGTT cleanup

This will make the code more readable, and extensible which is needed
for upcoming feature work. Eventually, we'll do the same for init.

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