minui: Support main fbdev connector in multi connector Blank function

Test: Power off device, observe display no longer stays on in offline charging
      mode after animation finished

Change-Id: I49b0fda43f0866583be04afb32214b2c2da2ed92
diff --git a/minui/graphics_fbdev.cpp b/minui/graphics_fbdev.cpp
index 4a7d325..2711af7 100644
--- a/minui/graphics_fbdev.cpp
+++ b/minui/graphics_fbdev.cpp
@@ -44,7 +44,11 @@
 }
 
 void MinuiBackendFbdev::Blank(bool blank, DrmConnector index) {
-  fprintf(stderr, "Unsupported multiple connectors, blank = %d, index = %d\n", blank, index);
+  if (index == DRM_MAIN) {
+    MinuiBackendFbdev::Blank(blank);
+  } else {
+    fprintf(stderr, "Unsupported multiple connectors, blank = %d, index = %d\n", blank, index);
+  }
 }
 
 bool MinuiBackendFbdev::HasMultipleConnectors() {