drm/omap: plane: Fix zpos initial value mismatch

While the omap_plane_init() function calls
drm_plane_create_zpos_property() with an initial value of 0,
omap_plane_reset() will force it to another value depending on the plane
type.

Fix the discrepancy by setting the initial zpos value to the same value
in the drm_plane_create_zpos_property() call.

Reviewed-by: Tomi Valkeinen <tomba@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221095918.18763-5-maxime@cerno.tech