drm/i915: don't disable DP port after a failed link training

Atm after a failed link training we disable the DP port. This can happen
during a modeset-enable or a DP link re-establishment. The latter can be
a problem and we shouldn't disable the DP port, see the previous patch for
the reasoning. In the former case the right thing would be to disable
the DP port, but also the rest of the pipe.

As a stop-gap solution leave the DP port enabled in both cases. It is an
improvement on its own (avoiding HW lock ups) and the proper solution
for the first case requires a bigger change, so let's keep that on the
TODO list.

v2:
- fix explanation of change impact (Chris)

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>