drm/tegra: Allow the driver to be built as a module

All APIs that the driver uses are exported, so the driver can now be
built as a module.
Signed-off-by: NThierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/Kconfig

 config DRM_TEGRA
-	bool "NVIDIA Tegra DRM"
+	tristate "NVIDIA Tegra DRM"
 	depends on ARCH_TEGRA || ARCH_MULTIPLATFORM
 	depends on DRM
 	depends on RESET_CONTROLLER