drm/i915: don't try to retrain a DP link on an inactive CRTC

Atm we may retrain the DP link even if the CRTC is inactive through
HPD work->intel_dp_check_link_status(). This in turn can lock up the PHY
(at least on BYT), since the DP port is disabled.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81948


Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org (3.16+)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>