drm/rockchip: remove vop_writel from vop1 driver

cppcheck reports
[drivers/gpu/drm/rockchip/rockchip_drm_vop.c:186]: (style) The function 'vop_writel' is never used.

vop_writel is static function that is not used, so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220521190716.1936193-1-trix@redhat.com