drm/i915: Disable SDVO port after the pipe on PCH platforms

While at it also remove the redundant/unneeded w/a like done for hdmi
already.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Mention that this also removes the unneeded w/a, as suggested
by Jesse.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>