drm/i915: Break after walking all GGTT vma in bump_inactive_ggtt

Since commit db6c2b41 ("drm/i915: Store the vma in an rbtree under
the object") the vma are once again sorted into GGTT first, then ppGTT
so that the typical case of walking the GGTT vma can stop as soon as we
find a non-ppGTT. Apply that optimisation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161223145804.6605-1-chris@chris-wilson.co.uk