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: 156bdac9

 ("drm/exynos: trigger build of all modules")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Inki Dae <daeinki@gmail.com>