ati-vga: Do not allow unaligned access via index register

ati-vga: Do not allow unaligned access via index register

According to docs bits 1 and 0 of MM_INDEX are hard coded to 0 so
unaligned access via this register should not be possible.
This also fixes problems reported in bug #1878134.

Buglink: https://bugs.launchpad.net/qemu/+bug/1878134
Signed-off-by: BALATON Zoltan <[email protected]>
Tested-by: Alexander Bulekov <[email protected]>
Acked-by: Alexander Bulekov <[email protected]>
Message-id: 20200516132352.39E9374594E@zero.eik.bme.hu
Signed-off-by: Gerd Hoffmann <[email protected]>