drm/rockchip: remove NR_LAYERS macro on vop2

There are 8 layers on rk3588, so a fix defined macro is
not appropriate.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20231013122051.1594164-1-andyshrk@163.com