hw/display: Rename VGA_ISA_MM -> VGA_MMIO

There is no ISA bus part in the MMIO VGA device, so rename:

 *  hw/display/vga-isa-mm.c -> hw/display/vga-mmio.c
 *  CONFIG_VGA_ISA_MM -> CONFIG_VGA_MMIO
 *  ISAVGAMMState -> VGAMmioState
 *  isa_vga_mm_init() -> vga_mmio_init()

Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211206224528.563588-2-f4bug@amsat.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>