[Intel-gfx] Revamp intel_sdvo and add missing properties

[Chris Wilson]

Some janitorial cleanups resulting in the addition of a few forgotten
properties for sdvo. As the cleanup touches many files within i915/ it is
likely to conflict with the various PLL bugfixes that are in the pipeline
and so I imagine will need resubmitting prior to merging. However, it will
be worth reviewing these preliminary patches. :)