drm/i915: Avoid spurious WARNs about the wrong pipe in the PPS code

drm/i915: Avoid spurious WARNs about the wrong pipe in the PPS code

Until recently vlv_steal_power_sequencer() wasn't being called for
normal DP ports, and hence it could assert that it should only be
called for pipe A and B (since pipe C doesn't support eDP). However
that changed when we started to consider normal DP ports as well when
choosing a PPS. So we will now get spurious warnings when
vlv_steal_power_sequencer() does get called for pipe C. Avoid this by
moving the WARN down into vlv_detach_power_sequencer() where this
assertion should still hold.

Cc: Imre Deak <imre.deak@intel.com>
Cc: stable@vger.kernel.org
Fixes: 5978e5140ce7 ("drm/i915: Prevent PPS stealing from a normal DP port on VLV/CHV")
References: https://bugs.freedesktop.org/show_bug.cgi?id=95287
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170208175254.10958-1-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>