drm/nouveau: store i2c port pointer directly in nouveau_encoder

This is about to become somewhat more complicated to determine in a
number of cases, so store the "common" case (DDC/AUX) directly inside
the encoder structure.

Pre-nv50 code not touched except to fill the pointer, don't care.

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