drm/i915: Only GGTT vma may be pinned and prevent shrinking

drm/i915: Only GGTT vma may be pinned and prevent shrinking

As only GGTT vma may be permanently pinned and are always at the head of
the object's vma list, as soon as we seen a ppGTT vma we can stop
searching for any_vma_pinned().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170525072528.11185-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>