drm/i915/gem: add missing else

Add missing else in set_proto_ctx_param() to fix coverity issue.

Addresses-Coverity: ("Unused value")
Fixes: d4433c7 ("drm/i915/gem: Use the proto-context to handle create parameters (v5)")
Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: katrinzhou <katrinzhou@tencent.com>
[tursulin: fixup alignment]
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220621124926.615884-1-tvrtko.ursulin@linux.intel.com

Author: KatrinJo

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -933,8 +933,9 @@ static int set_proto_ctx_param(struct drm_i915_file_private *fpriv,
 	case I915_CONTEXT_PARAM_PERSISTENCE:
 		if (args->size)
 			ret = -EINVAL;
-		ret = proto_context_set_persistence(fpriv->dev_priv, pc,
-						    args->value);
+		else
+			ret = proto_context_set_persistence(fpriv->dev_priv, pc,
+							    args->value);
 		break;
 
 	case I915_CONTEXT_PARAM_PROTECTED_CONTENT: