drm/nouveau: Fix nouveau_connector_ddc_detect()

It looks like that when we moved over to using
drm_connector_for_each_possible_encoder() in nouveau, that one rather
important part of this function got dropped by accident:

	/*          Right   v   here */
	for (i = 0; nv_encoder = NULL, i < DRM_CONNECTOR_MAX_ENCODER; i++) {
		int id = connector->encoder_ids[i];
		if (id == 0)
			break;

Since it's rather difficult to notice: the conditional in this loop is
actually:

	nv_encoder = NULL, i < DRM_CONNECTOR_MAX_ENCODER

Meaning that all early breaks result in nv_encoder keeping it's value,
otherwise nv_encoder = NULL. Ugh.

Since this got dropped, nouveau_connector_ddc_detect() now returns an
encoder for every single connector, regardless of whether or not it's
detected:

    [ 1780.056185] nouveau 0000:01:00.0: DRM: DDC responded, but no EDID for DP-2

So: fix this to ensure we only return an encoder if we actually found
one, and clean up the rest of the function while we're at it since it's
nearly impossible to read properly.

Changes since v1:
- Don't skip ddc probing for LVDS if we can't switch DDC through
  vga-switcheroo, just do the DDC probing without calling
  vga_switcheroo_lock_ddc() - skeggsb
Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: ddba766d ("drm/nouveau: Use drm_connector_for_each_possible_encoder()")
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_connector.c

@@ -412,9 +412,10 @@ nouveau_connector_ddc_detect(struct drm_connector *connector)
 	struct nouveau_connector *nv_connector = nouveau_connector(connector);
 	struct nouveau_drm *drm = nouveau_drm(dev);
 	struct nvkm_gpio *gpio = nvxx_gpio(&drm->client.device);
-	struct nouveau_encoder *nv_encoder = NULL;
+	struct nouveau_encoder *nv_encoder = NULL, *found = NULL;
 	struct drm_encoder *encoder;
-	int i, panel = -ENODEV;
+	int i, ret, panel = -ENODEV;
+	bool switcheroo_ddc = false;
 
 	/* eDP panels need powering on by us (if the VBIOS doesn't default it
 	 * to on) before doing any AUX channel transactions.  LVDS panel power
@@ -431,37 +432,43 @@ nouveau_connector_ddc_detect(struct drm_connector *connector)
 	drm_connector_for_each_possible_encoder(connector, encoder, i) {
 		nv_encoder = nouveau_encoder(encoder);
 
-		if (nv_encoder->dcb->type == DCB_OUTPUT_DP) {
-			int ret = nouveau_dp_detect(nv_encoder);
+		switch (nv_encoder->dcb->type) {
+		case DCB_OUTPUT_DP:
+			ret = nouveau_dp_detect(nv_encoder);
 			if (ret == NOUVEAU_DP_MST)
 				return NULL;
-			if (ret == NOUVEAU_DP_SST)
-				break;
-		} else
-		if ((vga_switcheroo_handler_flags() &
-		     VGA_SWITCHEROO_CAN_SWITCH_DDC) &&
-		    nv_encoder->dcb->type == DCB_OUTPUT_LVDS &&
-		    nv_encoder->i2c) {
-			int ret;
-			vga_switcheroo_lock_ddc(dev->pdev);
-			ret = nvkm_probe_i2c(nv_encoder->i2c, 0x50);
-			vga_switcheroo_unlock_ddc(dev->pdev);
-			if (ret)
+			else if (ret == NOUVEAU_DP_SST)
+				found = nv_encoder;
+
+			break;
+		case DCB_OUTPUT_LVDS:
+			switcheroo_ddc = !!(vga_switcheroo_handler_flags() &
+					    VGA_SWITCHEROO_CAN_SWITCH_DDC);
+		/* fall-through */
+		default:
+			if (!nv_encoder->i2c)
 				break;
-		} else
-		if (nv_encoder->i2c) {
+
+			if (switcheroo_ddc)
+				vga_switcheroo_lock_ddc(dev->pdev);
 			if (nvkm_probe_i2c(nv_encoder->i2c, 0x50))
-				break;
+				found = nv_encoder;
+			if (switcheroo_ddc)
+				vga_switcheroo_unlock_ddc(dev->pdev);
+
+			break;
 		}
+		if (found)
+			break;
 	}
 
 	/* eDP panel not detected, restore panel power GPIO to previous
 	 * state to avoid confusing the SOR for other output types.
 	 */
-	if (!nv_encoder && panel == 0)
+	if (!found && panel == 0)
 		nvkm_gpio_set(gpio, 0, DCB_GPIO_PANEL_POWER, 0xff, panel);
 
-	return nv_encoder;
+	return found;
 }
 
 static struct nouveau_encoder *