Revert "drm/i915: Disable SSC for outputs other than LVDS or DP"

This reverts commit 633f2ea2 and the
attempted fix dcbe6f2b.

There is a single clock source used for both SSC (some LVDS and DP) and
non-SSC (VGA, DVI) outputs. So we need to be careful to only enable SSC
as necessary. However, fiddling with DREFCLK was causing DP links to be
dropped and we do not have a fix ready, so revert.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>