vgabios: Don't use extra stack if it appears a modern OS is in use

vgabios: Don't use extra stack if it appears a modern OS is in use

If the last mode set (while not in vm86 mode) was done from a VBE mode
set call then disable the extra stack.  This works under the premise
that only a modern OS would invoke the VBE mode changing facilities
and a modern OS would always call the vgabios with sufficient stack
space.

This is an ugly hack to work around a problem Windows Vista (and
possibly later Windows releases) has with the VGA BIOS using a stack
in the e-segment.

Reported-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>