drm/v3d: add CONFIG_MMU dependency

Without CONFIG_MMU, we get a link error:

drivers/gpu/drm/v3d/v3d_bo.o: In function `v3d_gem_fault':
v3d_bo.c:(.text+0x3ca): undefined reference to `vm_insert_mixed'

The other drivers with this problem already depend on CONFIG_MMU,
so let's do the same thing here.

Fixes: 57692c94 ("drm/v3d: Introduce a new DRM driver for Broadcom V3D V3.x+")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525155030.3667352-5-arnd@arndb.de