drm/omap: plane: Remove redundant zpos initialisation

The omap KMS driver will call drm_plane_create_zpos_property() with an
init value of the plane index and the plane type.

Since the initial value wasn't carried over in the state, the driver had
to set it again in omap_plane_reset(). However, the helpers have been
adjusted to set it properly at reset, so this is not needed anymore.

Reviewed-by: Tomi Valkeinen <tomba@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221095918.18763-15-maxime@cerno.tech