drm/exynos: exynos_drm_ipp: fix return value check

In case of error, the function ipp_find_obj() 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: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>