[3.38] output/kms: Only add common modes for single mode connectors

If there was only a single mode, add the common modes to provide options to select other resolutions than the built in default. This avoids issues where the connector listed multiple supported modes, but where the common modes added would exceed the possible bandwidth. We could probably make an attempt to filter out more modes from the common mode list to avoid these issues, but it's likely that the driver already lists suitable modes, meaning there is no point in adding the common modes.

The common modes were initially added[0] to add modes to connectors with a single bundled mode, so we shouldn't regress the original bug fix.

[0] https://bugzilla.gnome.org/show_bug.cgi?id=744544

Closes: #1232 (closed)