macfb: set initial value of mode control registers in macfb_common_realize()

macfb: set initial value of mode control registers in macfb_common_realize()

If booting Linux directly in the q800 machine using -kernel rather than using a
MacOS toolbox ROM, the mode control registers are never initialised,
causing macfb_mode_write() to fail to determine the current resolution after
migration. Resolve this by always setting the initial values of the mode control
registers based upon the initial macfb properties during realize.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220305155530.9265-6-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>