drm/tegra: dc: Use devm_platform_ioremap_resource

The devm_platform_ioremap_resource() helper replaces few lines of a
boilerplate code with a single line, making code to look cleaner a tad.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>