drm/nouveau: fix ch7006 build

The ch7006 driver could be built even when nouveau was not enabled,
but the build fails in that case, so make it depend on DRM_NOUVEUA.

Also make the I2c encoder/helper chips menu depend on I2C (no build
error, just visual inspection).

ERROR: "drm_helper_probe_single_connector_modes" [drivers/gpu/drm/i2c/ch7006.ko] undefined!

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>