hw/mips/jazz: Inline vga_mmio_init() and remove it

vga_mmio_init() is used only one time and not very helpful,
inline and remove it.

Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211206224528.563588-5-f4bug@amsat.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>