FBC is already deactivated at this point.
Besides, nothing should be calling these lower-level function
pointers. A few months ago, the only caller of
dev_priv->fbc.deactivate was intel_pipe_set_base_atomic(), which was
the kgdboc function. But the following commit added it to the SKL
function:
    commit 
a8d201af68506b375b701d0d8dbe8487034256f2
    Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
    Date:   Thu Jan 7 11:54:11 2016 +0100
        drm/i915: Use plane state for primary plane updates.
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454101060-23198-1-git-send-email-paulo.r.zanoni@intel.com
 
        struct drm_i915_private *dev_priv = dev->dev_private;
        int pipe = to_intel_crtc(crtc)->pipe;
 
-       if (dev_priv->fbc.deactivate)
-               dev_priv->fbc.deactivate(dev_priv);
-
        I915_WRITE(PLANE_CTL(pipe, 0), 0);
        I915_WRITE(PLANE_SURF(pipe, 0), 0);
        POSTING_READ(PLANE_SURF(pipe, 0));