[media] vcodec: mediatek: Fix return value check in mtk_vcodec_init_enc_pm()

In case of error, the function devm_clk_get() returns ERR_PTR()
and not 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>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>