Add missing index that may have led us to enabling
more crtcs than necessary.

May also fix:
https://bugs.freedesktop.org/show_bug.cgi?id=56139Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>
Reviewed-by: NMichel Dänzer <michel.daenzer@amd.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>