drm/vmwgfx: Fix an error return check in vmw_compat_shader_add()

In vmw_compat_shader_add(), the return value check of vmw_shader_alloc()
is not proper. Modify the check for the return pointer 'res'.

Found by code review and compiled on ubuntu 20.04.

Fixes: 18e4a4669c50 ("drm/vmwgfx: Fix compat shader namespace")
Cc: [e-mail]
Signed-off-by: Haoxiang Li <[e-mail]>
Signed-off-by: Zack Rusin <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

drivers/gpu/drm/vmwgfx/vmwgfx_shader.c

···
 	ttm_bo_unreserve(&buf->tbo);
 
 	res = vmw_shader_alloc(dev_priv, buf, size, 0, shader_type);
-	if (unlikely(ret != 0))
+	if (IS_ERR(res)) {
+		ret = PTR_ERR(res);
 		goto no_reserve;
+	}
 
 	ret = vmw_cmdbuf_res_add(man, vmw_cmdbuf_res_shader,
 				 vmw_shader_key(user_key, shader_type),