nvc0: properly allocate copy engine class before using it

nvc0: properly allocate copy engine class before using it

Important for upcoming kernel changes to more correctly manage the CE
context on Volta and newer, or the channel will be killed in response
to a CTXNOTVALID error from the GPU.

The kernel will have a workaround for Volta and Turing GPUs to preserve
ABI, but will require userspace to behave correctly on Ampere and newer.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Acked-by: M Henning <drawoc@darkrefraction.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17633>