ati: use vga_read_byte in ati_cursor_define

This makes sure reads are confined to vga video memory.

v3: use uint32_t, fix cut+paste bug.
v2: fix ati_cursor_draw_line too.

Reported-by: xu hang <flier_m@outlook.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190917111441.27405-3-kraxel@redhat.com