drm/i915: Replace open-coding of DEFAULT_CONTEXT_ID

The intent of the check is made more clear if we use the proper name for
0 here.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -873,7 +873,7 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
 		break;
 	case I915_EXEC_BSD:
 		ring = &dev_priv->ring[VCS];
-		if (ctx_id != 0) {
+		if (ctx_id != DEFAULT_CONTEXT_ID) {
 			DRM_DEBUG("Ring %s doesn't support contexts\n",
 				  ring->name);
 			return -EPERM;
@@ -881,7 +881,7 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
 		break;
 	case I915_EXEC_BLT:
 		ring = &dev_priv->ring[BCS];
-		if (ctx_id != 0) {
+		if (ctx_id != DEFAULT_CONTEXT_ID) {
 			DRM_DEBUG("Ring %s doesn't support contexts\n",
 				  ring->name);
 			return -EPERM;
@@ -889,7 +889,7 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
 		break;
 	case I915_EXEC_VEBOX:
 		ring = &dev_priv->ring[VECS];
-		if (ctx_id != 0) {
+		if (ctx_id != DEFAULT_CONTEXT_ID) {
 			DRM_DEBUG("Ring %s doesn't support contexts\n",
 				  ring->name);
 			return -EPERM;