Feature: Add VRR_OFF Hint

[alex-courtis]

For a VRR unsupported monitor add a snippet to the warning message e.g.

HDMI-A-6: Cannot enable VRR, display or compositor may not support it.

To speed things up you can disable VRR for this display: add the following or similar to your cfg.yaml

VRR_OFF:
  - MAG274QRF-QD