drm/mtk: Use __drm_atomic_helper_crtc_reset

Now also comes with the added benefit of doing a drm_crtc_vblank_off(),
which means vblank state isn't ill-defined and fail-y at driver load
before the first modeset on each crtc.
Acked-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200612160056.2082681-4-daniel.vetter@ffwll.ch

drivers/gpu/drm/mediatek/mtk_drm_crtc.c

@@ -113,19 +113,15 @@ static void mtk_drm_crtc_reset(struct drm_crtc *crtc)
 {
 	struct mtk_crtc_state *state;
 
-	if (crtc->state) {
+	if (crtc->state)
 		__drm_atomic_helper_crtc_destroy_state(crtc->state);
 
-		state = to_mtk_crtc_state(crtc->state);
-		memset(state, 0, sizeof(*state));
-	} else {
-		state = kzalloc(sizeof(*state), GFP_KERNEL);
-		if (!state)
-			return;
-		crtc->state = &state->base;
-	}
+	kfree(to_mtk_crtc_state(crtc->state));
+	crtc->state = NULL;
 
-	state->base.crtc = crtc;
+	state = kzalloc(sizeof(*state), GFP_KERNEL);
+	if (state)
+		__drm_atomic_helper_crtc_reset(crtc, &state->base);
 }
 
 static struct drm_crtc_state *mtk_drm_crtc_duplicate_state(struct drm_crtc *crtc)