Don't change the max_bpc connector prop if mode=current.

As things are, even when mode=current is specified on the .ini file,
a full modeset is needed (and done), which causes a very noticeable
screen blinking. That is because setting the max_bpc on a connector
needs full modesetting.
The idea here is that if mode=current on the .ini, no modesetting
should be done, so the current max_bpc is programmed into the
connector.
But if a custom max-bpc=... is specified, that will be used instead,
even if mode=current on the .ini

Fixes: #660



Signed-off-by: vanfanel <redwindwanderer@gmail.com>