drm/gpuvm: add drm_gpuvm_flags to drm_gpuvm

Introduce flags for struct drm_gpuvm, this required by subsequent
commits.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231108001259.15123-8-dakr@redhat.com