[recipes] Compute DM flags in gen_tasks

Change-Id: I3d82d78161373ec0536cbe6df196c753547501a8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/274200
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Ben Wagner aka dogben <benjaminwagner@google.com>
64 files changed