drm/nv50: prevent accidently turning off encoders we're actually using

On most cards the DisplayPort connector is created with 2 encoders sharing
a single SOR (for native DP, and for DVI-over-DP).  The previous logic
for turning off unused encoders didn't take into account that we could
have multiple drm_encoders on a single hw encoder and ended up turning off
encoders that were actually being used still.

This patch fixes that issue.  We probably want to look at something a bit
better later on, and only expose one drm_encoder per hw encoder block.

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