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

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 <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Cc: [email protected] (3.16+)
Signed-off-by: Jani Nikula <[email protected]>