drm/exynos: add CONFIG_MMU dependency

Compile-testing this driver on a NOMMU configuration shows a link failure:

drivers/gpu/drm/exynos/exynos_drm_gem.o: In function `exynos_drm_gem_fault':
exynos_drm_gem.c:(.text+0x484): undefined reference to `vmf_insert_mixed'

Add a CONFIG_MMU dependency to ensure we only enable this in configurations
that build correctly.

Many other drm drivers have the same dependency. It would be nice to
make this work in MMU-less configurations, but evidently nobody has
ever needed this so far.

Fixes: 156bdac99061 ("drm/exynos: trigger build of all modules")
Signed-off-by: Arnd Bergmann <[e-mail]>
Reviewed-by: Vladimir Murzin <[e-mail]>
Signed-off-by: Inki Dae <[e-mail]>

drivers/gpu/drm/exynos/Kconfig

···
 config DRM_EXYNOS
 	tristate "DRM Support for Samsung SoC EXYNOS Series"
 	depends on OF && DRM && (ARCH_S3C64XX || ARCH_S5PV210 || ARCH_EXYNOS || ARCH_MULTIPLATFORM || COMPILE_TEST)
+	depends on MMU
 	select DRM_KMS_HELPER
 	select VIDEOMODE_HELPERS
 	select SND_SOC_HDMI_CODEC if SND_SOC