drm/rockchip: Remove unused variable

Commit 977697e2 ("drm/atomic: Pass the full state to planes atomic
disable and update") added the old_state variable instead of what used
to be a parameter, but it also removed the sole user of that variable in
the vop_plane_atomic_update function leading to an usused variable.
Remove it.

Fixes: 977697e2 ("drm/atomic: Pass the full state to planes atomic disable and update")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210319152920.262035-1-maxime@cerno.tech