drm/i915: Serialise with remote retirement

Since retirement may be running in a worker on another CPU, it may be
skipped in the local intel_gt_wait_for_idle(). To ensure the state is
consistent for our sanity checks upon load, serialise with the remote
retirer by waiting on the timeline->mutex.

Outside of this use case, e.g. on suspend or module unload, we expect the
slack to be picked up by intel_gt_pm_wait_for_idle() and so prefer to
put the special case serialisation with retirement in its single user,
for now at least.

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/20191121071044.97798-2-chris@chris-wilson.co.uk
(cherry picked from commit 2d0fb251)
Fixes: 093b9228 ("drm/i915: Split i915_active.mutex into an irq-safe spinlock for the rbtree")
Closes: https://gitlab.freedesktop.org/drm/intel/issues/754


Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>