drm/ttm: refine ghost BO resv criteria

Ghost BOs need to stick with the resv object only when the origin is imported.

This is a low hanging fruit to avoid OOM situations on evictions.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: xinhui pan <xinhui.pan@amd.com>
Link: https://patchwork.freedesktop.org/patch/352740/