fb: cfb_shell: turn off display_blanking at initializing

Turn off display_blanking to make it able to refresh on a device
even when blanking default off.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>
diff --git a/subsys/fb/cfb_shell.c b/subsys/fb/cfb_shell.c
index bede0cd..7f0fcda 100644
--- a/subsys/fb/cfb_shell.c
+++ b/subsys/fb/cfb_shell.c
@@ -416,6 +416,12 @@
 		return -ENODEV;
 	}
 
+	err = display_blanking_off(dev);
+	if (err) {
+		shell_error(shell, "Failed to turn off display blanking: %d", err);
+		return err;
+	}
+
 	err = cfb_framebuffer_init(dev);
 	if (err) {
 		shell_error(shell, "Framebuffer initialization failed!");