There is already a dependency on CXL_REGION, which depends on CXL_BUS,
but since CXL_REGION is a 'bool' symbol, it's possible to configure
DAX as built-in even though CXL itself is a loadable module:

x86_64-linux-ld: drivers/dax/cxl.o: in function `cxl_dax_region_probe':
cxl.c:(.text+0xb): undefined reference to `to_cxl_dax_region'
x86_64-linux-ld: drivers/dax/cxl.o: in function `cxl_dax_region_driver_init':
cxl.c:(.init.text+0x10): undefined reference to `__cxl_driver_register'
x86_64-linux-ld: drivers/dax/cxl.o: in function `cxl_dax_region_driver_exit':
cxl.c:(.exit.text+0x9): undefined reference to `cxl_driver_unregister'

Prevent this with another depndency on the tristate symbol.

Fixes: 09d09e04 ("cxl/dax: Create dax devices for CXL RAM regions")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230214103054.1082908-1-arnd@kernel.orgSigned-off-by: Dan Williams <dan.j.williams@intel.com>