drm/nv50: prevent switching off SOR when in use for DVI-over-DP

Another hack because of us exposing each encoder block's function as
an encoder rather than exposing a single encoder that deals with them
all.

A proper fix will come, it's just rather invasive so this hack will
do until then.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>