macfb: fix a memory leak (CID 1465231)

Rewrite the function using g_string_append_printf() rather than
g_strdup_printf()/g_strconcat().

Fixes: df8abbba ("macfb: add common monitor modes supported by the MacOS toolbox ROM")
Cc: mark.cave-ayland@ilande.co.uk
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20211105165254.3544369-1-laurent@vivier.eu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>