drm/i915: Unpin stolen pages

The way the stolen handling works is we take a pin on the backing pages,
but we never actually get a reference to the bo. On freeing objects
allocated with stolen memory, the final unref will end up freeing the
object with pinned pages count left. To enable an assertion to catch
bugs in this code path, this patch cleans up that remaining pin.

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>