macfb: implement mode sense to allow display type to be detected

macfb: implement mode sense to allow display type to be detected

The MacOS toolbox ROM uses the monitor sense to detect the display type and then
offer a fixed set of resolutions and colour depths accordingly. Implement the
monitor sense using information found in Apple Technical Note HW26: "Macintosh
Quadra Built-In Video" along with some local experiments.

Since the default configuration is 640 x 480 with 8-bit colour then hardcode
the sense register to return MACFB_DISPLAY_VGA for now.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20211007221253.29024-8-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>