drm/nouveau/gsp: remove gsp-specific chid allocation path

In order to specify a channel ID to RM during channel allocation, the
channel ID is broken down into a "userd page" index and an index into
that page.

It was assumed that RM would enforce that the same physical block of
memory be used for all CHIDs within a "userd page", and the GSP paths
override NVKM's normal CHID allocation to handle this.

However, none of that turns out to be necessary.

Remove the GSP-specific code and use the regular CHID allocation path.

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