drm/ttm: move ghost object creation to a common function

Both accel cleanup and pipeline move had the same code, make
a single function for it.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200917043040.146575-7-airlied@gmail.com