drm/rockchip: vop2: rename grf to sys_grf

The vop2 need to reference more grf(system grf, vop grf, vo0/1 grf,etc)
in the upcoming rk3588.

So we rename the current system grf to sys_grf.

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/20231211115826.1785190-1-andyshrk@163.com