drm/atmel-hlcdc: Allow async page flips

The driver is capable of doing async page flips so we need to tell the
core to allow them.

Signed-off-by: Dan Sneddon <dan.sneddon@microchip.com>
Tested-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210330151721.6616-1-dan.sneddon@microchip.com

drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.c

@@ -593,6 +593,7 @@ static int atmel_hlcdc_dc_modeset_init(struct drm_device *dev)
 	dev->mode_config.max_width = dc->desc->max_width;
 	dev->mode_config.max_height = dc->desc->max_height;
 	dev->mode_config.funcs = &mode_config_funcs;
+	dev->mode_config.async_page_flip = true;
 
 	return 0;
 }