drm/ttm: use the parent resv for ghost objects v3

This way the TTM is destroyed with the correct dma_resv object
locked and we can even pipeline imported BO evictions.

v2: Limit this to only cases when the parent object uses a separate
    reservation object as well. This fixes another OOM problem.
v3: fix init and try_lock on the wrong object

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.freedesktop.org/patch/337499/