vgabios: Avoid memory references via %esp register in vgabios.

Win7 interprets the vgabios and it does not properly handle memory
references relative to the %esp register.  Commit 4a8b58cb introduced
some of these %esp references - rework that assembler to avoid these
instructions and make win7 happy.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>