gpu: drm: img: Replace system work flush with img dedicated workqueue

gpu: drm: img: Replace system work flush with img dedicated workqueue

To remove deprecated flush_scheduled_work() after the commit
20bdedafd2f6 ("workqueue: Warn attempt to flush system-wide workqueues."),
add img dedicated unordered wq and queue/flush the wq instead of
system work.

Ref: 848a4e5c096d ("drm/i915: add a dedicated workqueue inside drm_i915_private")
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: I20d872118f906033e4b93e7a730580933a4cb8b6