init: always initialize device-mapper

Device-mapper is required already, so drop the conditionals for trying to
load it.  GetRootEntry() was depending on its existance, so this adds the
required dependency.

Bug: 131747478
Test: boot without any verity/avb/dm-linear devices.
Change-Id: Ifc89d3b338e2c2b8faaf1c24e2792c1eeb5bbbde