drm/i915/guc: Fix outstanding G2H accounting

A small race that could result in incorrect accounting of the number
of outstanding G2H. Basically prior to this patch we did not increment
the number of outstanding G2H if we encoutered a GT reset while sending
a H2G. This was incorrect as the context state had already been updated
to anticipate a G2H response thus the counter should be incremented.

As part of this change we remove a legacy (now unused) path that was the
last caller requiring a G2H response that was not guaranteed to loop.
This allows us to simplify the accounting as we don't need to handle the
case where the send fails due to the channel being busy.

Also always use helper when decrementing this value.

v2 (Daniele): update GEM_BUG_ON check, pull in dead code removal from
later patch, remove loop param from context_deregister.

Fixes: f4eb1f3fe946 ("drm/i915/guc: Ensure G2H response has space in buffer")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210909164744.31249-3-matthew.brost@intel.com

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