Restore TW_SCREEN_BLANK_ON_BOOT functionality.

This is required for a number of devices, particularly now that custom
graphics support is deprecated.

This change is required to officially support TWRP on the Huawei Plank
(aka the Honor 7).

Change-Id: Ie16ec557c75654c9c9e7a2877aee3339fc214d05

minuitwrp/graphics.cpp

@@ -360,6 +360,12 @@ int gr_init(void)
     gr_flip();
     gr_flip();
 
+#ifdef TW_SCREEN_BLANK_ON_BOOT
+    printf("TW_SCREEN_BLANK_ON_BOOT := true\n");
+    gr_fb_blank(true);
+    gr_fb_blank(false);
+#endif
+
     return 0;
 }