drm/i915: Remove i915_request.lock requirement for execution callbacks

We are using the i915_request.lock to serialise adding an execution
callback with __i915_request_submit. However, if we use an atomic
llist_add to serialise multiple waiters and then check to see if the
request is already executing, we can remove the irq-spinlock.

v2: Avoid using the irq_work when outside of the irq-spinlocks, where we
can execute the callbacks immediately.
v3: Pay close attention to the order of setting ACTIVE on retirement, we
need to ensure the request is signaled and breadcrumbs detached before
we finish removing the request from the engine.

Fixes: 1d9221e9 ("drm/i915: Skip signaling a signaled request")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716142207.13003-2-chris@chris-wilson.co.uk


Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>