The macfb VRAM memory region was configured with coalescing rather than dirty
memory logging enabled, causing some areas of the screen not to redraw after
a full screen update.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Fixes: 8ac919a0 ("hw/m68k: add Nubus macfb video card")
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220108164147.30813-1-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>