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)

Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>