drm/nouveau: move usercopy helpers to nouveau_drv.h

Move the usercopy helpers to a common driver header file to make it
usable for the new API added in subsequent commits.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230804182406.5222-7-dakr@redhat.com