[recipes] Move DM flags logic into gen_tasks_logic/dm_flags.go

Change-Id: I294495f952c8310bf1518363778fda9690c8c83e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/274202
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Ben Wagner aka dogben <benjaminwagner@google.com>
3 files changed