hw/display/apple-gfx: Adds configurable mode list

hw/display/apple-gfx: Adds configurable mode list

This change adds a property 'display_modes' on the graphics device
which permits specifying a list of display modes. (screen resolution
and refresh rate)

The property is an array of a custom type to make the syntax slightly
less awkward to use, for example:

-device '{"driver":"apple-gfx-pci", "display-modes":["1920x1080@60", "3840x2160@60"]}'

Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Tested-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-ID: <20241223221645.29911-5-phil@philjordan.eu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>