firmware: raspberrypi: Add RPI_FIRMWARE_NOTIFY_DISPLAY_DONE

The RPI_FIRMWARE_NOTIFY_DISPLAY_DONE firmware call allows to tell the
firmware the kernel is in charge of the display now and the firmware can
free whatever resources it was using.

Acked-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20211215095117.176435-2-maxime@cerno.tech