drm/i915/sdvo: convert to encoder disable/enable

Similar to crt, this doesn't convert the dpms functions.
Also similar to crt, we don't switch of the display pipe
for the intermediate modes, only DPMS_OFF is truely off.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>