drm/exynos/decon5433: kill BIT_CLKS_ENABLED flag

The flag was used to check if IRQ handlers can touch HW. Since driver
enables IRQs only if hardware is enabled the flag becomes redundant.

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