drm/exynos: crtc: rework atomic_{begin,flush}

Some CRTC drivers (like Exynos DRM Mixer) can handle blocking register
updates only on per-device level, not per-plane level. This patch changes
exynos_crts atomic_begin/atomic_flush callbacks to handle the entire crtc,
instead of given planes, so driver can handle both cases on their own.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>