drm/qxl: Remove deprecated create_singlethread_workqueue

System workqueues have been able to handle high level of concurrency
for a long time now and there's no reason to use dedicated workqueues
just to gain concurrency. Since the workqueue in the QXL graphics device
driver is involved in freeing and processing the release ring
(workitem &qdev->gc_workqxl, maps to gc_work which calls
qxl_garbage_collect) and is not being used on a memory reclaim path,
dedicated gc_queue has been replaced with the use of system_wq.

Unlike a dedicated per-cpu workqueue created with create_workqueue(),
system_wq allows multiple work items to overlap executions even on
the same CPU; however, a per-cpu workqueue doesn't have any CPU
locality or global ordering guarantees unless the target CPU is
explicitly specified and thus the increase of local concurrency
shouldn't make any difference.

flush_work() has been called in qxl_device_fini() to ensure that there
are no pending tasks while disconnecting the driver.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160702110209.GA3560@Karyakshetra