drm/i915: Mark the GuC log buffer flush interrupts handling WQ as freezable

drm/i915: Mark the GuC log buffer flush interrupts handling WQ as freezable

The GuC log buffer flush work item has to do a register access to send the
ack to GuC and this work item, if not synced before suspend, can potentially
get executed after the GFX device is suspended. This work item function uses
rpm get/put calls around the Hw access, which covers the rpm suspend case
but for system suspend a sync would be required as kernel can potentially
schedule the work items even after some devices, including GFX, have been
put to suspend. But sync has to be done only for the system suspend case,
as sync along with rpm get/put can cause a deadlock for rpm suspend path.
To have the sync, but like a NOOP, for rpm suspend path also this work
item could have been queued from the irq handler only when the device is
runtime active & kept active while that work item is pending or getting
executed but an interrupt can come even after the device is out of use and
so can potentially lead to missing of this work item.

By marking the workqueue, dedicated for handling GuC log buffer flush
interrupts, as freezable we don't have to bother about flushing of this
work item from the suspend hooks, the pending work item if any will be
either executed before the suspend or scheduled later on resume. This way
the handling of log buffer flush work item can be kept same between system
suspend & rpm suspend.

Suggested-by: Imre Deak <imre.deak@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>