drm/ttm/tests: Require MMU when testing

Satisfy MMU dependency when testing TTM with KUnit. This fixes
compilation errors on platforms that don't select this option
by default.

Signed-off-by: Karolina Stolarek <karolina.stolarek@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308110133.f0lhFwMV-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202308111032.enU8IisR-lkp@intel.com/
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230811113649.697886-1-karolina.stolarek@intel.com
Signed-off-by: Christian König <christian.koenig@amd.com>