drm/atomic: shutdown *current* encoder

In disable_outputs() we need to shut down the outgoing encoder, not the
incoming one (we have already swapped-state at this point).  Without
this, we end up telling the driver to crtc->dpms(OFF) without first
encoder->dpms(OFF), and that makes some hw quite unhappy.

v2: missing WARN_ON() hunk and comment

Reviewed-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>