drm/i915/ddi: Avoid long delays during system suspend / eDP disabling

Atm disabling either DP or eDP outputs can generate a spurious short
pulse interrupt. The reason is that after disabling the port the source
will stop sending a valid stream data, while the sink expects either a
valid stream or the idle pattern. Since neither of this is sent the sink
assumes (after an arbitrary delay) that the link is lost and requests
for link retraining with a short pulse.

The spurious pulse is a real problem at least for eDP panels with long
power-off / power-cycle delays: as part of disabling the output we
disable the panel power. The subsequent spurious short pulse handling
will have to turn the power back on, which means the driver has to do a
redundant wait for the power-off and power-cycle delays. During system
suspend this leads to an unnecessary delay up to ~1s on systems with
such panels as reported by Rui.

To fix this put the sink to DPMS D3 state before turning off the port.
According to the DP spec in this state the sink should not request
retraining. This is also what we do already on pre-ddi platforms.

As an alternative I also tried configuring the port to send idle pattern
- which is against BSPec - and leave the port in normal mode before
turning off the port. Neither of these resolved the problem.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-and-tested-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496250335-7627-1-git-send-email-imre.deak@intel.com