drm/sun4i: dsi: Use NULL to signify "no panel"

The continued use of an ERR_PTR to signify "no panel" outside of
sun6i_dsi_attach is confusing because it is a double negative. Because
the connector always reports itself as connected, there is also the
possibility of sending an ERR_PTR to drm_panel_get_modes(), which would
crash.

Solve both of these by only storing the panel pointer if it is valid.

Fixes: 133add5b ("drm/sun4i: Add Allwinner A31 MIPI-DSI controller support")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211072858.30784-2-samuel@sholland.org

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c

@@ -748,7 +748,7 @@ static void sun6i_dsi_encoder_enable(struct drm_encoder *encoder)
 	phy_configure(dsi->dphy, &opts);
 	phy_power_on(dsi->dphy);
 
-	if (!IS_ERR(dsi->panel))
+	if (dsi->panel)
 		drm_panel_prepare(dsi->panel);
 
 	/*
@@ -763,7 +763,7 @@ static void sun6i_dsi_encoder_enable(struct drm_encoder *encoder)
 	 * ordering on the panels I've tested it with, so I guess this
 	 * will do for now, until that IP is better understood.
 	 */
-	if (!IS_ERR(dsi->panel))
+	if (dsi->panel)
 		drm_panel_enable(dsi->panel);
 
 	sun6i_dsi_start(dsi, DSI_START_HSC);
@@ -779,7 +779,7 @@ static void sun6i_dsi_encoder_disable(struct drm_encoder *encoder)
 
 	DRM_DEBUG_DRIVER("Disabling DSI output\n");
 
-	if (!IS_ERR(dsi->panel)) {
+	if (dsi->panel) {
 		drm_panel_disable(dsi->panel);
 		drm_panel_unprepare(dsi->panel);
 	}
@@ -941,11 +941,13 @@ static int sun6i_dsi_attach(struct mipi_dsi_host *host,
 			    struct mipi_dsi_device *device)
 {
 	struct sun6i_dsi *dsi = host_to_sun6i_dsi(host);
+	struct drm_panel *panel = of_drm_find_panel(device->dev.of_node);
 
+	if (IS_ERR(panel))
+		return PTR_ERR(panel);
+
+	dsi->panel = panel;
 	dsi->device = device;
-	dsi->panel = of_drm_find_panel(device->dev.of_node);
-	if (IS_ERR(dsi->panel))
-		return PTR_ERR(dsi->panel);
 
 	dev_info(host->dev, "Attached device %s\n", device->name);