q800: wire macfb IRQ to separate video interrupt on VIA2

Whilst the in-built Quadra 800 framebuffer exists within the Nubus address
space for slot 9, it has its own dedicated interrupt on VIA2. Force the
macfb device to occupy slot 9 in the q800 machine and wire its IRQ to the
separate video interrupt since this is what is expected by the MacOS
interrupt handler.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20211007221253.29024-14-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>