drm/i915: Allow sharing the idle-barrier from other kernel requests

drm/i915: Allow sharing the idle-barrier from other kernel requests

By placing our idle-barriers in the i915_active fence tree, we expose
those for reuse by other components that are issuing requests along the
kernel_context. Reusing the proto-barrier active_node is perfectly fine
as the new request implies a context-switch, and so an opportune point
to run the idle-barrier. However, the proto-barrier is not equivalent
to a normal active_node and care must be taken to avoid dereferencing the
ERR_PTR used as its request marker.

v2: Comment the more egregious cheek
v3: A glossary!

Reported-by: Lionel Landwerlin <[email protected]>
Fixes: ce476c80b8bf ("drm/i915: Keep contexts pinned until after the next kernel context switch")
Fixes: a9877da2d629 ("drm/i915/oa: Reconfigure contexts on the fly")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Lionel Landwerlin <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]