compositor-drm: Calculate atomic-commit flags earlier

Shift up our calculation of the flags we use for atomic commits. We will
later use this to differentiate between test-only and full commits when
printing debug information inside drm_output_state_apply_atomic.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>