drm/i915: Implement SINGLE_TIMELINE with a syncobj (v4)

This API is entirely unnecessary and I'd love to get rid of it.  If
userspace wants a single timeline across multiple contexts, they can
either use implicit synchronization or a syncobj, both of which existed
at the time this feature landed.  The justification given at the time
was that it would help GL drivers which are inherently single-timeline.
However, neither of our GL drivers actually wanted the feature.  i965
was already in maintenance mode at the time and iris uses syncobj for
everything.

Unfortunately, as much as I'd love to get rid of it, it is used by the
media driver so we can't do that.  We can, however, do the next-best
thing which is to embed a syncobj in the context and do exactly what
we'd expect from userspace internally.  This isn't an entirely identical
implementation because it's no longer atomic if userspace races with
itself by calling execbuffer2 twice simultaneously from different
threads.  It won't crash in that case; it just doesn't guarantee any
ordering between those two submits.  It also means that sync files
exported from different engines on a SINGLE_TIMELINE context will have
different fence contexts.  This is visible to userspace if it looks at
the obj_name field of sync_fence_info.

Moving SINGLE_TIMELINE to a syncobj emulation has a couple of technical
advantages beyond mere annoyance.  One is that intel_timeline is no
longer an api-visible object and can remain entirely an implementation
detail.  This may be advantageous as we make scheduler changes going
forward.  Second is that, together with deleting the CLONE_CONTEXT API,
we should now have a 1:1 mapping between intel_context and
intel_timeline which may help us reduce locking.

v2 (Tvrtko Ursulin):
 - Update the comment on i915_gem_context::syncobj to mention that it's
   an emulation and the possible race if userspace calls execbuffer2
   twice on the same context concurrently.
v2 (Jason Ekstrand):
 - Wrap the checks for eb.gem_context->syncobj in unlikely()
 - Drop the dma_fence reference
 - Improved commit message

v3 (Jason Ekstrand):
 - Move the dma_fence_put() to before the error exit

v4 (Tvrtko Ursulin):
 - Add a comment about fence contexts to the commit message

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-8-jason@jlekstrand.net