drm/i915: Hold onto an explicit ref to i915_vma_work.pinned

Since __vma_release is run by a kworker after the fence has been
signaled, it is no longer protected by the active reference on the vma,
and so the alias of vw->pinned to vma->obj is also not protected by a
reference on the object. Add an explicit reference for vw->pinned so it
will always be safe.

Found by inspection.

Fixes: 54d7195f ("drm/i915: Unpin vma->obj on early error")
Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: <stable@vger.kernel.org> # v5.6+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201102161931.30031-1-chris@chris-wilson.co.uk