drm/i915/gt: Pull release of node->age under the spinlock

We need to ensure that the list is valid prior to marking the node as
retrievable, otherwise we may see two threads compete over the same node
in intel_gt_get_buffer_pool(). If the first thread acquires and releases
the node in the same jiffie, the second thread may then acquire it (as
the jiffie now again matches the expected value) and claim the node
before it is put back into the list.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730134049.8822-1-chris@chris-wilson.co.uk
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>