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=56139
Signed-off-by: Alex Deucher <alexander.deucher@amd.com> Reviewed-by: Michel Dänzer <michel.daenzer@amd.com> Signed-off-by: Dave Airlie <airlied@redhat.com>