media: mtk-mdp: use IS_ERR to check return value of of_clk_get

Function of_clk_get() returns an ERR_PTR on failures. In file
mtk_mdp_commp.c, its return value is checked against NULL. Such checks
cannot prevent from accessing bad memory. This patch replaces the NULL
checks with IS_ERR checks.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>