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>

hw/display/macfb.c

@@ -655,6 +655,14 @@ static bool macfb_common_realize(DeviceState *dev, MacfbState *s, Error **errp)
         return false;
     }
 
+    /*
+     * Set mode control registers to match the mode found above so that
+     * macfb_mode_write() does the right thing if no MacOS toolbox ROM
+     * is present to initialise them
+     */
+    s->regs[DAFB_MODE_CTRL1 >> 2] = s->mode->mode_ctrl1;
+    s->regs[DAFB_MODE_CTRL2 >> 2] = s->mode->mode_ctrl2;
+
     s->con = graphic_console_init(dev, 0, &macfb_ops, s);
     surface = qemu_console_surface(s->con);