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>

drivers/gpu/drm/tegra/dc.c

@@ -957,6 +957,7 @@ static struct drm_plane *tegra_dc_cursor_plane_create(struct drm_device *drm,
 	}
 
 	drm_plane_helper_add(&plane->base, &tegra_cursor_plane_helper_funcs);
+	drm_plane_create_zpos_immutable_property(&plane->base, 255);
 
 	return &plane->base;
 }