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

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

@@ -188,12 +188,6 @@ struct vop {
 	struct vop_win win[];
 };
 
-static inline void vop_writel(struct vop *vop, uint32_t offset, uint32_t v)
-{
-	writel(v, vop->regs + offset);
-	vop->regsbak[offset >> 2] = v;
-}
-
 static inline uint32_t vop_readl(struct vop *vop, uint32_t offset)
 {
 	return readl(vop->regs + offset);