drm/radeon: properly track the crtc not_enabled case evergreen_mc_stop()

commit 804cc4a0 upstream.

The save struct is not initialized previously so explicitly
mark the crtcs as not used when they are not in use.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>