drm/i915/guc: reserve the doorbell before selecting the cacheline

Cacheline selection is only needed if we actually manage to reserve a
doorbell.

Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181022230427.5616-2-daniele.ceraolospurio@intel.com

drivers/gpu/drm/i915/intel_guc_submission.c

@@ -955,6 +955,10 @@ guc_client_alloc(struct drm_i915_private *dev_priv,
 	}
 	client->vaddr = vaddr;
 
+	ret = reserve_doorbell(client);
+	if (ret)
+		goto err_vaddr;
+
 	client->doorbell_offset = __select_cacheline(guc);
 
 	/*
@@ -967,10 +971,6 @@ guc_client_alloc(struct drm_i915_private *dev_priv,
 	else
 		client->proc_desc_offset = (GUC_DB_SIZE / 2);
 
-	ret = reserve_doorbell(client);
-	if (ret)
-		goto err_vaddr;
-
 	DRM_DEBUG_DRIVER("new priority %u client %p for engine(s) 0x%x: stage_id %u\n",
 			 priority, client, client->engines, client->stage_id);
 	DRM_DEBUG_DRIVER("doorbell id %u, cacheline offset 0x%lx\n",