drm/i915: Reset power sequencer pipe tracking when disp2d is off

The power sequencer loses its state when the disp2d power well is down.
Clear the dev_priv->pps_pipe tracking so that the power sequencer state
gets reinitialized the next time it's needed.

v2: Fix the pps_mutex vs. power_domain mutex deadlock by taking power
    domain reference first
v3: Rename from edp_pps_(un)lock() to just pps_(un)lock() for the future,
    update due to backlight code changes

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>