drm/i915: New lock to serialize the Host2GuC actions
With the addition of new Host2GuC actions related to GuC logging, there
is a need of a lock to serialize them, as they can execute concurrently
with each other and also with other existing actions.
v2: Use mutex in place of spinlock to serialize, as sleep can happen
while waiting for the action's response from GuC. (Tvrtko)
v3: To conform to the general rules, acquire mutex before taking the
forcewake. (Tvrtko)