drm/nouveau: uvmm: remove dedicated VM pointer from VMAs

VMAs can find their corresponding VM through their embedded struct
drm_gpuva which already carries a pointer to a struct drm_gpuva_manager
which the VM is based on. Hence, remove the struct nouveau_uvmm pointer
from struct nouveau_uvma to save a couple of bytes per mapping.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230807163238.2091-6-dakr@redhat.com