drm/exynos/decon5433: kill BIT_IRQS_ENABLED flag

Since DECON uses enable_irq/disable_irq to full control IRQs,
there is no point in having flags to trace it separately.
As a bonus condition for software trigger becomes always true,
so it can be removed.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>