drm/i915: don't deactivate FBC at skylake_disable_primary_plane

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 a8d201af
    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

drivers/gpu/drm/i915/intel_display.c

@@ -3156,9 +3156,6 @@ static void skylake_disable_primary_plane(struct drm_plane *primary,
 	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));