drm/i915: Call encoder->post_disable() in intel_sanitize_encoder()

VLV and CHV disable the DP port only in the .post_disable() hook, so we
need to make intel_sanitize_encoder() call that when it's trying to
disable encoders without an active pipes.

My bsw actaully hits this when an external display is connected. The
BIOS still likes to turn on the eDP port, but leaves the pipe disabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rafael Barbalho <rafael.barbalho@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>