drm/i915/guc: Selftest for stealing of guc ids

Testing the stealing of guc ids is hard from user space as we have 64k
guc_ids. Add a selftest, which artificially reduces the number of guc
ids, and forces a steal.

The test creates a spinner which is used to block all subsequent
submissions until it completes. Next, a loop creates a context and a NOP
request each iteration until the guc_ids are exhausted (request creation
returns -EAGAIN). The spinner is ended, unblocking all requests created
in the loop. At this point all guc_ids are exhausted but are available
to steal. Try to create another request which should successfully steal
a guc_id. Wait on last request to complete, idle GPU, verify a guc_id
was stolen via a counter, and exit the test. Test also artificially
reduces the number of guc_ids so the test runs in a timely manner.

v2:
 (John Harrison)
  - s/stole/stolen
  - Fix some wording in test description
  - Rework indexing into context array
  - Add test description to commit message
  - Fix typo in commit message
 (Checkpatch)
  - s/guc/(guc) in NUMBER_MULTI_LRC_GUC_ID
v3:
 (John Harrison)
  - Set array value to NULL after extracting error
  - Fix a few typos in comments / error messages
  - Delete redundant comment in commit message

Signed-off-by: Matthew Brost <[email protected]>
Reviewed-by: John Harrison <[email protected]>
Signed-off-by: John Harrison <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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