The driver's VDD on/off logic assumes that whenever the VDD is on we
also hold an AUX power domain reference. Since BIOS can leave the VDD on
during booting and resuming and on DDI platforms we won't take a
corresponding power reference, the above assumption won't hold on those
platforms and an eventual delayed VDD off work will do an extraneous AUX
power domain put resulting in a refcount underflow. Fix this the same
way we did this for non-DDI DP encoders:

commit 6d93c0c4 ("drm/i915: fix VDD state tracking after system
resume")

At the same time call the DP encoder suspend handler the same way as the
non-DDI DP encoders do to flush any pending VDD off work. Leaving the
work running may cause a HW access where we don't expect this (at a point
where power domains are suspended already).

While at it remove an unnecessary function call indirection.

This fixed for me AUX refcount underflow problems on BXT during
suspend/resume.

CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
CC: stable@vger.kernel.org
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/1460963062-13211-4-git-send-email-imre.deak@intel.com
(cherry picked from commit bf93ba67)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>