drm/tegra: Increase compile test coverage

The ARCH_MULTIPLATFORM dependency was introduced back when Tegra didn't
support multiplatform yet as a means to allow the driver to be easily
compile-tested along with other DRM drivers. In the meantime, the new
COMPILE_TEST Kconfig option has been introduced for exactly that
purpose, so use that instead to clarify the intention.

Signed-off-by: Thierry Reding <treding@nvidia.com>