PDISP doesn't like it when disabled CRTCs are poked.

Fixes external output not coming to life when it has cursor on.
https://bugs.freedesktop.org/show_bug.cgi?id=41608Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>