drm/exynos/mixer: replace MXR_INT_EN register cache with flag

Driver uses only VSYNC interrupts, so we need to cache VSYNC bit state only.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>