drm/tegra: Mark nvdec PM functions as __maybe_unused

The resume helper is called conditionally and causes a harmless warning
when stubbed out:

drivers/gpu/drm/tegra/nvdec.c:240:12: error: 'nvdec_runtime_resume' defined but not used [-Werror=unused-function]
  240 | static int nvdec_runtime_resume(struct device *dev)

Mark both suspend and resume as __maybe_unused for consistency to avoid
this warning.

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