drm/hisilicon: Fix return value check in ade_dts_parse()

In case of error, the function devm_clk_get() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Chen Feng <puck.chen@hisilicon.com>