milkymist: Move DT_NOGRAPHIC check outside milkymist_tmu2_create()

DT_NOGRAPHIC handling will be moved to a MachineState field, and
it will be easier to change milkymist_init() to check that field.

Cc: Michael Walle <michael@walle.cc>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
