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

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

@@ -879,8 +879,6 @@ static void vop_plane_atomic_disable(struct drm_plane *plane,
 static void vop_plane_atomic_update(struct drm_plane *plane,
 		struct drm_atomic_state *state)
 {
-	struct drm_plane_state *old_state = drm_atomic_get_old_plane_state(state,
-									   plane);
 	struct drm_plane_state *new_state = drm_atomic_get_new_plane_state(state,
 									   plane);
 	struct drm_crtc *crtc = new_state->crtc;