drm/rockchip: vop2: fix null pointer in plane_atomic_disable

If the vop2_plane_atomic_disable function is called with NULL as a
state, accessing the old_pstate runs into a null pointer exception.
However, the drm_atomic_helper_disable_planes_on_crtc function calls the
atomic_disable callback with state NULL.

Allow to disable a plane without passing a plane state by checking the
old_pstate only if a state is passed.
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221028095206.2136601-2-m.tretter@pengutronix.de

drivers/gpu/drm/rockchip/rockchip_drm_vop2.c

@@ -996,13 +996,15 @@ static int vop2_plane_atomic_check(struct drm_plane *plane,
 static void vop2_plane_atomic_disable(struct drm_plane *plane,
 				      struct drm_atomic_state *state)
 {
-	struct drm_plane_state *old_pstate = drm_atomic_get_old_plane_state(state, plane);
+	struct drm_plane_state *old_pstate = NULL;
 	struct vop2_win *win = to_vop2_win(plane);
 	struct vop2 *vop2 = win->vop2;
 
 	drm_dbg(vop2->drm, "%s disable\n", win->data->name);
 
-	if (!old_pstate->crtc)
+	if (state)
+		old_pstate = drm_atomic_get_old_plane_state(state, plane);
+	if (old_pstate && !old_pstate->crtc)
 		return;
 
 	vop2_win_disable(win);