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

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

For CRT and SDVO/HDMI, we need to use a normal, non-SSC, clock and so we
must clear any enabling bits left-over from earlier outputs. And also
seems to correct the LVDS panel on the Lenovo U160.

However, at one point, it did cause an "ERROR failed to disable
trancoder". So prolonged testing on top of Jesse's refactored and
error-checking CRTC logic is desired.

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