drm/vmwgfx: Fix a buffer object leak

A buffer object leak was introduced when fixing a premature buffer
object release. Fix this.

Cc: <stable@vger.kernel.org>
Fixes: 73a88250b7 ("Fix a destoy-while-held mutex problem.")
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_kms.c

@@ -2595,6 +2595,7 @@ void vmw_kms_helper_resource_finish(struct vmw_validation_ctx *ctx,
 		vmw_kms_helper_buffer_finish(res->dev_priv, NULL, ctx->buf,
 					     out_fence, NULL);
 
+	vmw_dmabuf_unreference(&ctx->buf);
 	vmw_resource_unreserve(res, false, NULL, 0);
 	mutex_unlock(&res->dev_priv->cmdbuf_mutex);
 }