drm/tegra: Mark nvdec_writel() as inline

Without CONFIG_IOMMU_API, the nvdec_writel() function is unused, causing
a warning:

drivers/gpu/drm/tegra/nvdec.c:48:13: error: 'nvdec_writel' defined but not used [-Werror=unused-function]
   48 | static void nvdec_writel(struct nvdec *nvdec, u32 value, unsigned int offset)
      |             ^~~~~~~~~~~~

As this is a trivial wrapper around an inline function, mark it as
inline itself, which avoids the warning as well.

Fixes: e76599df354d ("drm/tegra: Add NVDEC driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>