Enable ddcutil support by default

It was disabled 5 years ago due to issues that were not written down, in commit 687587ea.

Things have improved since then, both in libddc and in powerdevil (eg: cbd6d0ea), so we might want to give this a try.

Caveats:

• We still don't have support for N>1 separate brightness values, not here nor in the plasmoid. It is something we might want to add after this.
• libddc requires the user to be in the i2c group for it to work, which I'm not sure how we can achieve from plasma.