drm/gpuvm: allow building as module

Currently, the DRM GPUVM does not have any core dependencies preventing
a module build.

Also, new features from subsequent patches require helpers (namely
drm_exec) which can be built as module.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230920144343.64830-3-dakr@redhat.com