drm/nouveau/dp: check for NULL nv_connector->native_mode

Add checking for NULL before calling nouveau_connector_detect_depth() in
nouveau_connector_get_modes() function because nv_connector->native_mode
could be dereferenced there since connector pointer passed to
nouveau_connector_detect_depth() and the same value of
nv_connector->native_mode is used there.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: d4c2c99b ("drm/nouveau/dp: remove broken display depth function, use the improved one")
Signed-off-by: Natalia Petrova <n.petrova@fintech.ru>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230512111526.82408-1-n.petrova@fintech.ru

drivers/gpu/drm/nouveau/nouveau_connector.c

@@ -966,7 +966,7 @@ nouveau_connector_get_modes(struct drm_connector *connector)
 	/* Determine display colour depth for everything except LVDS now,
 	 * DP requires this before mode_valid() is called.
 	 */
-	if (connector->connector_type != DRM_MODE_CONNECTOR_LVDS)
+	if (connector->connector_type != DRM_MODE_CONNECTOR_LVDS && nv_connector->native_mode)
 		nouveau_connector_detect_depth(connector);
 
 	/* Find the native mode if this is a digital panel, if we didn't
@@ -987,7 +987,7 @@ nouveau_connector_get_modes(struct drm_connector *connector)
 	 * "native" mode as some VBIOS tables require us to use the
 	 * pixel clock as part of the lookup...
 	 */
-	if (connector->connector_type == DRM_MODE_CONNECTOR_LVDS)
+	if (connector->connector_type == DRM_MODE_CONNECTOR_LVDS && nv_connector->native_mode)
 		nouveau_connector_detect_depth(connector);
 
 	if (nv_encoder->dcb->type == DCB_OUTPUT_TV)