drm/xe: Introduce an xe_validation wrapper around drm_exec

Introduce a validation wrapper xe_validation_guard() as a helper
intended to be used around drm_exec transactions what perform
validations. Once TTM can handle exhaustive eviction we could
remove this wrapper or make it mostly a NO-OP unless other
functionality is added to it.

Currently the wrapper takes a read lock upon entry and if the
transaction hits an OOM, all locks are released and the
transaction is retried with a write-lock. If all other
validations participate in this scheme, the transaction with
the write lock will be the only transaction validating and
should have access to all available non-pinned memory.

There is currently a problem in that TTM converts -EDEADLOCKS to
-ENOMEM, and with ww_mutex slowpath error injections, we can hit
-ENOMEMs without having actually ran out of memory. We abuse
ww_mutex internals to detect such situations until TTM is fixes
to not convert the error code. In the meantime, injecting
ww_mutex slowpath -EDEADLOCKs is a good way to test
the implementation in the absence of real OOMs.

Just introduce the wrapper in this commit. It will be hooked up
to the driver in following commits.

v2:
- Mark class_xe_validation conditional so that the loop is
  skipped on initialization error.
- Argument sanitation (Matt Brost)
- Fix conditional execution of xe_validation_ctx_fini()
  (Matt Brost)
- Add a no_block mode for upcoming use in the CPU fault handler.
v4:
- Update kerneldoc. (Xe CI).

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250908101246.65025-3-thomas.hellstrom@linux.intel.com

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