drm/i915: Handle recursive shrinker for vma->last_active allocation

If we call into the shrinker for direct relcaim inside kmalloc, it will
retire the requests. If we retire the vma->last_active while processing a
new i915_vma_move_to_active() we can upset the delicate bookkeeping
required for the cache. After the possible invocation of the shrinker, we
need to double check the vma->last_active is still valid.

Fixes: 8b293eb5

 ("drm/i915: Track the last-active inside the i915_vma")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105600#c39
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180719072206.16015-1-chris@chris-wilson.co.uk