gpu: host1x: 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>

drivers/gpu/host1x/Kconfig

 config TEGRA_HOST1X
 	tristate "NVIDIA Tegra host1x driver"
-	depends on ARCH_TEGRA || ARCH_MULTIPLATFORM
+	depends on ARCH_TEGRA || (ARM && COMPILE_TEST)
 	help
 	  Driver for the NVIDIA Tegra host1x hardware.