drm/i915/guc: remove writes to GEN8_DRBREG registers

These registers are not actually writable by the CPU; only the GuC can
actually program them. So let's not do writes that have no effect.

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>

drivers/gpu/drm/i915/i915_guc_submission.c

@@ -252,14 +252,9 @@ static void guc_disable_doorbell(struct intel_guc *guc,
 
 	doorbell->db_status = GUC_DOORBELL_DISABLED;
 
-	I915_WRITE(drbreg, I915_READ(drbreg) & ~GEN8_DRB_VALID);
-
 	value = I915_READ(drbreg);
 	WARN_ON((value & GEN8_DRB_VALID) != 0);
 
-	I915_WRITE(GEN8_DRBREGU(client->doorbell_id), 0);
-	I915_WRITE(drbreg, 0);
-
 	/* XXX: wait for any interrupts */
 	/* XXX: wait for workqueue to drain */
 }