drm/nouveau: separately allocate struct nouveau_uvmm

Allocate struct nouveau_uvmm separately in preparation for subsequent
commits introducing reference counting for struct drm_gpuvm.

While at it, get rid of nouveau_uvmm_init() as indirection of
nouveau_uvmm_ioctl_vm_init() and perform some minor cleanups.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231108001259.15123-9-dakr@redhat.com