drm/i915/guc: Handle race condition where wakeref count drops below 0

There is a rare race condition when preparing for a reset where
guc_lrc_desc_unpin() could be in the process of deregistering a context
while a different thread is scrubbing outstanding contexts and it alters
the context state and does a wakeref put. Then, if there is a failure
with deregister_context(), a second wakeref put could occur. As a result
the wakeref count could drop below 0 and fail an INTEL_WAKEREF_BUG_ON()
check.

Therefore if there is a failure with deregister_context(), undo the
context state changes and do a wakeref put only if the context was set
to be destroyed earlier.

v2: Expand comment to better explain change. (Daniele)
v3: Removed addition to the original comment. (Daniele)

Fixes: 2f2cc53b5fe7 ("drm/i915/guc: Close deregister-context race against CT-loss")
Signed-off-by: Jesus Narvaez <[email protected]>
Cc: Daniele Ceraolo Spurio <[email protected]>
Cc: Alan Previn <[email protected]>
Cc: Anshuman Gupta <[email protected]>
Cc: Mousumi Jana <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Cc: Matt Roper <[email protected]>
Reviewed-by: Daniele Ceraolo Spurio <[email protected]>
Signed-off-by: John Harrison <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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