drm/ttm: Document and optimize ttm_bo_pipeline_gutting()

If the bo is idle when calling ttm_bo_pipeline_gutting(), we unnecessarily
create a ghost object and push it out to delayed destroy.
Fix this by adding a path for idle, and document the function.

Also avoid having the bo end up in a bad state vulnerable to user-space
triggered kernel BUGs if the call to ttm_tt_create() fails.

Finally reuse ttm_bo_pipeline_gutting() in ttm_bo_evict().

Cc: Christian König <[email protected]>
Signed-off-by: Thomas Hellström <[email protected]>
Reviewed-by: Christian König <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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