drm/i915: Introduce i915_vm_to_ggtt()

In a couple of places, we have an i915_address_space that we know is
really an i915_ggtt that we want to use. Create an inline helper to
convert from the i915_address_space subclass into its container.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461833819-3991-4-git-send-email-chris@chris-wilson.co.uk