staging: omapdrm: use alloc_ordered_workqueue() instead of UNBOUND w/ max_active = 1

This is an equivalent conversion and will ease scheduled removal of
WQ_NON_REENTRANT.

Only compile tested.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>