defconfig: gen3auto: Enable dm-init module

From the kernel 5.1, added a "dm-mod.create" kernle cmdline parameter,
which allows device-mapper targets to be configured at boot time.
This enables early use of DM targets in the boot process (as the root
device or otherwise) without the need of an initramfs. So enabled the
dm-init module for dm-verity.

Change-Id: Ice0c62d00f49f8c038fd315c8baf076fa83c5daf
Signed-off-by: Nan Zhang <nanzhan@codeaurora.org>