drm/exynos: preset zpos value for overlay planes

Usually userspace don't want to have two overlay planes on the same zpos
so this change assign a different zpos for each plane. Before this change
a zpos of value zero was created for all planes so the userspace had to
set up the zpos of every plane it wanted to use.

Also all places that were storing zpos positions are now unsigned int.

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