drm: bump DRM_CONNECTOR_MAX_ENCODER from 2 to 3

There exists at least one NVIDIA GPU (Quadro NVS 300) that has a DMS-59
connector which is capable of supporting DisplayPort, TMDS and VGA on
a single connector.

We need to bump the allowed encoder limit to support all three configs.

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