drm/tegra: Include <linux/of.h>

Include <linux/of.h> to get the contained declarations. No functional
changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Thierry Reding <treding@nvidia.com>