drm/i915/guc: don't spinwait if the GuC's workqueue is full
Rather than wait to see whether more space becomes available in the GuC
submission workqueue, we can just return -EAGAIN and let the caller try
again in a little while. This gets rid of an uninterruptable sleep in
the polling code :)
We'll also add a counter to the GuC client statistics, to see how often
we find the WQ full.
v2:
Flag the likely() code path (Tvtrko Ursulin).
v4:
Add/update comments about failure counters (Tvtrko Ursulin).
Signed-off-by: Dave Gordon <david.s.gordon@intel.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>