drm/radeon: on hotplug force link training to happen (v2)

To have DP behave like VGA/DVI we need to retrain the link
on hotplug. For this to happen we need to force link
training to happen by setting connector dpms to off
before asking it turning it on again.

v2: agd5f
- drop the dp_get_link_status() change in atombios_dp.c
  for now.  We still need the dpms OFF change.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>