drm/nouveau/kms/nv50-: Use state helper instead of crtc pointer

dispnv50 references the crtc->state pointer in order to get the current
CRTC state in its atomic_check hook, which would be the old CRTC state in
the global atomic state.

Use the drm_atomic_get_old_crtc_state helper to get that state to make it
more obvious.
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201102133834.1176740-1-maxime@cerno.tech

drivers/gpu/drm/nouveau/dispnv50/head.c

@@ -313,11 +313,13 @@ nv50_head_atomic_check_mode(struct nv50_head *head, struct nv50_head_atom *asyh)
 static int
 nv50_head_atomic_check(struct drm_crtc *crtc, struct drm_atomic_state *state)
 {
+	struct drm_crtc_state *old_crtc_state = drm_atomic_get_old_crtc_state(state,
+									      crtc);
 	struct drm_crtc_state *crtc_state = drm_atomic_get_new_crtc_state(state,
 									  crtc);
 	struct nouveau_drm *drm = nouveau_drm(crtc->dev);
 	struct nv50_head *head = nv50_head(crtc);
-	struct nv50_head_atom *armh = nv50_head_atom(crtc->state);
+	struct nv50_head_atom *armh = nv50_head_atom(old_crtc_state);
 	struct nv50_head_atom *asyh = nv50_head_atom(crtc_state);
 	struct nouveau_conn_atom *asyc = NULL;
 	struct drm_connector_state *conns;