drm/i915/guc: drop cached copy of 'wq_head'

Now that we keep the GuC client process descriptor permanently mapped,
we don't really need to keep a local copy of the GuC's work-queue-head.
So we can simplify the code a little by not doing this.

Signed-off-by: Alex Dai <yu.dai@intel.com>
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>