drm/exynos: use atomic helper commit

This patch replaces specific atomic commit function
with atomic helper commit one.

For this, it removes existing atomic commit function
and relevant code specific to Exynos DRM and makes
atomic helper commit to be used instead.

Below are changes for the use of atomic helper commit:
- add atomic_commit_tail callback specific to Exynos DRM
  . default implemention of atomic helper doesn't mesh well
    with runtime PM so the device driver which supports runtime
    PM should call drm_atomic_helper_commit_modeset_enables function
    prior to drm_atomic_helper_commit_planes function call.
    atomic_commit_tail callback implements this call ordering.
- allow plane commit only in case that CRTC device is enabled.
  . for this, it calls atomic_helper_commit_planes function
    with DRM_PLANE_COMMIT_ACTIVE_ONLY flag in atomic_commit_tail callback.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>