drm/rockchip: Fix typo in VOP macros argument

Fix a small typo in the macros VOP argument. The macro argument
is currently wrongly named "x", and then never used. The code
built fine almost by accident, as the macros are always used
in a context where a proper "vop" symbol exists.

This fix is almost cosmetic, as the resulting code shouldn't change.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20190109185639.5093-2-ezequiel@collabora.com