Backport S/VGA timing fix from 86Box

https://github.com/86Box/86Box/commit/b67c3c41e15ba4357d248b7f2e79118a57bb76aa

The off by one in PCem causes the refresh rate for S/VGA to be wrong (mainly relevant for DOS).

For example, standard DOS 70Hz (25.175MHz clock, 8 pixels per character, 95 Horizontal Total, 449 scanlines)
`(25175000/((95+6)*8))/449 -> 69.3923791043Hz`
`(25175000/((95+5)*8))/449 -> 70.0863028953Hz`