drm/exynos: use irq_flags instead of triggering

The drm_handle_vblank should be called whenever be vsync, te interrupt
means vsync on i80 interface.

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