drm/tegra: Fix return value check

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

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Thierry Reding <treding@nvidia.com>