video/exynos: don't use [delayed_]work_pending()

There's no need to test whether a (delayed) work item in pending
before queueing, flushing or cancelling it.  Most uses are unnecessary
and quite a few of them are buggy.

Remove unnecessary pending tests from exynos_dp_core.  Only compile
tested.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>