drm/i915: Replace call to commit_planes_on_crtc with internal update, v2.

drm_atomic_helper_commit_planes_on_crtc calls begin_commit,
then plane_update hooks, then flush_commit. Because we keep our own
visibility tracking through plane_state->visible there's no need to
rely on the atomic hooks for this.

By explicitly writing our own helper, we can update visible planes
as needed, which is useful to make NV12 support work as intended.

Changes since v1:
- Reword commit message. (Matt Roper)
- Rename to intel_update_planes_on_crtc(). (Matt)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180920102711.4184-6-maarten.lankhorst@linux.intel.com