Don't change the max_bpc connector prop if mode=current and no max-bpc...

I think this is importante because, 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. And 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

-If --use-current-max-bpc is passed on the commandline, the current max_bpc will be programmed into the connector, no matter what the .ini says.

Hope it's clear, thanks.

Fixes: #660 (closed)