drm/msm: Use per-ring submit lists

To make it easier to determine what submit(s) are currently active
on any given ring, store the active submits in per-ring lists instead
of a master list.

Change-Id: Ic0dedbadb331cd8e4e85dfcfa51cb1ceabe6efa9
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>