drm/i915/fbc: don't try to deactivate FBC if it's not enabled

During FBC invalidation, don't call intel_fbc_deactivate if it's not
enabled. This doesn't fix any bug, but helps making the interface
saner.
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/1453210558-7875-21-git-send-email-paulo.r.zanoni@intel.com

drivers/gpu/drm/i915/intel_fbc.c

@@ -982,7 +982,7 @@ void intel_fbc_invalidate(struct drm_i915_private *dev_priv,
 
 	fbc->busy_bits |= intel_fbc_get_frontbuffer_bit(fbc) & frontbuffer_bits;
 
-	if (fbc->busy_bits)
+	if (fbc->enabled && fbc->busy_bits)
 		intel_fbc_deactivate(dev_priv);
 
 	mutex_unlock(&fbc->lock);