drivers/dax: Allow to include DEV_DAX_PMEM as builtin

This move the dependency to DEV_DAX_PMEM_COMPAT such that only
if DEV_DAX_PMEM is built as module we can allow the compat support.

This allows to test the new code easily in a emulation setup where we
often build things without module support.

Cc: <stable@vger.kernel.org>
Fixes: 730926c3 ("device-dax: Add /sys/class/dax backwards compatibility")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>