Enable drm core zpos normalization for planes.

Changes in v3:
- None
Signed-off-by: NJeykumar Sankaran <jsanka@codeaurora.org>
Reviewed-by: NSean Paul <seanpaul@chromium.org>
Signed-off-by: NSean Paul <seanpaul@chromium.org>
Signed-off-by: NRob Clark <robdclark@gmail.com>