drm/object: Add default zpos value at reset

The drm_plane_create_zpos_property() function asks for an initial value,
and will set the associated plane state variable with that value if a
state is present.

However, that function is usually called at a time where there's no
state yet. Since the drm_plane_state reset helper doesn't take care of
reading that value when it's called, it means that in most cases the
initial value will be 0, or the drivers will have to work around it.

Let's add some code in __drm_atomic_helper_plane_state_reset() to get
the initial zpos value if the property has been attached in order to fix
this.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221095918.18763-8-maxime@cerno.tech