[x86 setup] Don't rely on the VESA BIOS being register-clean

The VESA BIOS is specified to be register-clean.  However, we have now
found at least one system which violates that.  Thus, be as paranoid
about VESA calls as about everything else.

Huge thanks to Will Simoneau for reporting, diagnosing, and testing
this out on Dell Inspiron 5150.

Cc: Will Simoneau <simoneau@ele.uri.edu>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>