drm/i915/fbc: move intel_fbc_{enable, disable} call one level up

Instead of duplicating the calls for every platform, let's just put
them in the correct places inside intel_atomic_commit. This will also
make it easier for us to move the enable call in order to support
fasbtoot.

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/1453210558-7875-19-git-send-email-paulo.r.zanoni@intel.com
diff --git a/drivers/gpu/drm/i915/intel_fbc.c b/drivers/gpu/drm/i915/intel_fbc.c
index 5bf7f84..60644dd 100644
--- a/drivers/gpu/drm/i915/intel_fbc.c
+++ b/drivers/gpu/drm/i915/intel_fbc.c
@@ -1137,7 +1137,7 @@
 	WARN_ON(!mutex_is_locked(&fbc->lock));
 	WARN_ON(!fbc->enabled);
 	WARN_ON(fbc->active);
-	assert_pipe_disabled(dev_priv, crtc->pipe);
+	WARN_ON(crtc->active);
 
 	DRM_DEBUG_KMS("Disabling FBC on pipe %c\n", pipe_name(crtc->pipe));