drm/bridge: parade-ps8640: Link device to ensure suspend/resume order

Entering suspend while the display attached to this bridge is still on
makes the resume sequence to resume the bridge first, display last:
when this happens, we get a timeout while resuming the bridge, as its
MCU will get stuck due to the display being unpowered.

On the other hand, on mt8173-elm, closing the lid makes the display to
get powered off first, bridge last, so at resume time the sequence is
swapped (compared to the first example) and everything just works
as expected.

Add a stateless device link to the DRM device that this bridge belongs
to, ensuring a correct resume sequence and solving the unability to
correctly resume bridge operation in the first mentioned example.
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220105090802.73564-1-angelogioacchino.delregno@collabora.comReviewed-by: Robert Foss <robert.foss@linaro.org>

drivers/gpu/drm/bridge/parade-ps8640.c

@@ -102,6 +102,7 @@ struct ps8640 {
 	struct regulator_bulk_data supplies[2];
 	struct gpio_desc *gpio_reset;
 	struct gpio_desc *gpio_powerdown;
+	struct device_link *link;
 	bool pre_enabled;
 };
 
@@ -456,14 +457,36 @@ static int ps8640_bridge_attach(struct drm_bridge *bridge,
 		return ret;
 	}
 
+	ps_bridge->link = device_link_add(bridge->dev->dev, dev, DL_FLAG_STATELESS);
+	if (!ps_bridge->link) {
+		dev_err(dev, "failed to create device link");
+		ret = -EINVAL;
+		goto err_devlink;
+	}
+
 	/* Attach the panel-bridge to the dsi bridge */
-	return drm_bridge_attach(bridge->encoder, ps_bridge->panel_bridge,
-				 &ps_bridge->bridge, flags);
+	ret = drm_bridge_attach(bridge->encoder, ps_bridge->panel_bridge,
+				&ps_bridge->bridge, flags);
+	if (ret)
+		goto err_bridge_attach;
+
+	return 0;
+
+err_bridge_attach:
+	device_link_del(ps_bridge->link);
+err_devlink:
+	drm_dp_aux_unregister(&ps_bridge->aux);
+
+	return ret;
 }
 
 static void ps8640_bridge_detach(struct drm_bridge *bridge)
 {
-	drm_dp_aux_unregister(&bridge_to_ps8640(bridge)->aux);
+	struct ps8640 *ps_bridge = bridge_to_ps8640(bridge);
+
+	drm_dp_aux_unregister(&ps_bridge->aux);
+	if (ps_bridge->link)
+		device_link_del(ps_bridge->link);
 }
 
 static struct edid *ps8640_bridge_get_edid(struct drm_bridge *bridge,