drm/tegra: Add zpos property for cursor planes

As of commit 4dc55525 ("drm: plane: Verify that no or all planes
have a zpos property") a warning is emitted if there's a mix of planes
with and without a zpos property.

On Tegra, cursor planes are always composited on top of all other
planes, which is why they never had a zpos property attached to them.
However, since the composition order is fixed, this is trivial to
remedy by simply attaching an immutable zpos property to them.

v3: do not hardcode zpos for overlay planes used as cursor (Dmitry)
v2: hardcode cursor plane zpos to 255 instead of 0 (Ville)

Reported-by: Jonathan Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>