drm/i915: don't disable_fbc() if FBC is already disabled

If FBC is disabled we will still call intel_fbc_invalidate(), and as a
result we may call intel_fbc_deactivate(), which will try to touch
registers.

I'm pretty sure I saw this happen on a runtime suspended device, and
I'm almost sure I was running igt/pm_rpm. It produced the "you touched
registers while the device is suspended" WARNs. But this was some time
ago and I can't remember exactly which conditions were necessary to
reproduce the problem.

v2: Rebase to new series order.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446664257-32012-8-git-send-email-paulo.r.zanoni@intel.comSigned-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/intel_fbc.c

@@ -438,7 +438,8 @@ static void __intel_fbc_disable(struct drm_i915_private *dev_priv)
 
 	intel_fbc_cancel_work(dev_priv);
 
-	dev_priv->fbc.disable_fbc(dev_priv);
+	if (dev_priv->fbc.enabled)
+		dev_priv->fbc.disable_fbc(dev_priv);
 	dev_priv->fbc.crtc = NULL;
 }