macfb: don't use special irq_state and irq_mask variables in MacfbState

The current IRQ state and IRQ mask are handled exactly the same as standard
register accesses, so store these values directly in the regs array rather
than having separate variables for them.

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: <20220305155530.9265-3-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>