drm/exynos: factor out initial setting of each driver

From fimd driver and vidi driver, dev->irq_enabled and
dev->vblank_disable_allowed are set and also mixer needs them even if
missed. It's duplicated so set them when loads drm driver.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>