vga: fix banked access bounds checking (CVE-2016-3710)

vga allows banked access to video memory using the window at 0xa00000
and it supports a different access modes with different address
calculations.

The VBE bochs extentions support banked access too, using the
VBE_DISPI_INDEX_BANK register.  The code tries to take the different
address calculations into account and applies different limits to
VBE_DISPI_INDEX_BANK depending on the current access mode.

Which is probably effective in stopping misprogramming by accident.
But from a security point of view completely useless as an attacker
can easily change access modes after setting the bank register.

Drop the bogus check, add range checks to vga_mem_{readb,writeb}
instead.

Fixes: CVE-2016-3710
Reported-by: Qinghao Tang <luodalongde@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
