drm/exynos/decon5433: refactor irq requesting code

To allow runtime validation of mode of work irq request
code should be split into two separate phases:
- irq reqesting,
- irq checking.
Following patches will move 2nd phase to mode validation phase.

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