driver: Set panel prepare_prev_first

When using DRM_PANEL_BRIDGE we want the previous bridge (the DSI host)
to prepare before the panel, so that we are able to send DSI commands
to the panel. Without this flag submitting DSI commands fails.

Author: stephan-gh

driver.py

@@ -374,6 +374,7 @@ def generate_probe(p: Panel, options: Options) -> str:
 
 	drm_panel_init(&ctx->panel, dev, &{p.short_id}_panel_funcs,
 		       DRM_MODE_CONNECTOR_DSI);
+	ctx->panel.prepare_prev_first = true;
 '''
 
 	if options.backlight_fallback_dcs: