Atm we setup the HW panel power sequencer logic both for eDP and DP
ports. On eDP we then go on and start the power on sequence and commence
with link training when it's ready. On DP we don't do the power on
sequencing but do the link training immediately. At this point the DP
PHY block gets stuck, since - supposedly - it is waiting for the power
on sequence to finish. The actual register write that seems to hold off
the PHY is PIPEX_PP_ON_DELAYS[Panel Control Port Select]. Writing here
a non-0 value eventually sets PIPEX_PP_STATUS[Require Asset Status] to
1 and blocks the PHY until the panel power on is ready.

Fix this by not doing any PP sequencing setup for DP ports.

Thanks to Ville Syrjälä, Jesse Barnes and Todd Previte for the help in
tracking this down.

Note that on older gmch platforms (where we have lvds instead of edp)
we've hacked around this by writing the magic ABCD unlock key to PP
registers, which disables the hw sanity checks.

For edp all platforms thus far had the pch split, with the edp port in
the north display complex and the PP registers on the pch the hw
sanity checks (expressed through the "Require Asset Status" bit) was
never functional, hence never a real issue.

This regression has been introduce in

commit bf13e81b
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Fri Sep 6 07:40:05 2013 +0300

    drm/i915: add support for per-pipe power sequencing on vlv
Signed-off-by: Imre Deak <imre.deak@intel.com>
[danvet: Add note about the bigger story here.]
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>