OMAP: DSS2: DSI: Use system workqueue for delayed work instead of a private workqueue

In the previous DSI driver design, a private workqueue was needed to prevent a
deadlock as explained in the commit : 0f16aa0a
. In the current design, the workqueue is only used for queueing delayed work in
the case where we don't get a FRAMEDONE interrupt for 250 milliseconds. It is
safe to remove the private workqueue amd use the system workqueue instead to
schedule the delayed work with the new design where the deadlock can't occur.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>