drm/i915/guc: Skip port assign on first iteration of GuC dequeue

If port[0] is occupied and we're trying to dequeue request from
different context, we will inevitably hit BUG_ON in port_assign.
Let's skip it - similar to what we're doing in execlists counterpart.

Fixes: 77f0d0e ("drm/i915/execlists: Pack the count into the low bits of the port.request")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Wajdeczko <michal.wajdeczko@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170523102400.9614-2-michal.winiarski@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_guc_submission.c

@@ -681,7 +681,8 @@ static bool i915_guc_dequeue(struct intel_engine_cs *engine)
 					goto done;
 				}
 
-				port_assign(port, last);
+				if (submit)
+					port_assign(port, last);
 				port++;
 			}