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