drivers: display_dummy: implement display_flush

Implements the `display_flush` API function, introduced in zephyrproject-rtos#79936.

Signed-off-by: Martin Stumpf <martin.stumpf@vected.de>

drivers/display/display_dummy.c

@@ -57,6 +57,11 @@ static int dummy_display_write(const struct device *dev, const uint16_t x,
 	return 0;
 }
 
+static int dummy_display_flush(const struct device *dev)
+{
+	return 0;
+}
+
 static int dummy_display_blanking_off(const struct device *dev)
 {
 	return 0;
@@ -110,6 +115,7 @@ static const struct display_driver_api dummy_display_api = {
 	.blanking_on = dummy_display_blanking_on,
 	.blanking_off = dummy_display_blanking_off,
 	.write = dummy_display_write,
+	.flush = dummy_display_flush,
 	.set_brightness = dummy_display_set_brightness,
 	.set_contrast = dummy_display_set_contrast,
 	.get_capabilities = dummy_display_get_capabilities,