drm/i915/guc: Ensure request ordering via completion fences

If two requests are on the same ring, they are explicitly ordered by the
HW. So, a submission fence is sufficient to ensure ordering when using
the new GuC submission interface. Conversely, if two requests share a
timeline and are on the same physical engine but different context this
doesn't ensure ordering on the new GuC submission interface. So, a
completion fence needs to be used to ensure ordering.

v2:
 (Daniele)
  - Don't delete spin lock
v3:
 (Daniele)
  - Delete forward dec

Signed-off-by: John Harrison <[email protected]>
Signed-off-by: Matthew Brost <[email protected]>
Reviewed-by: Daniele Ceraolo Spurio <[email protected]>
Signed-off-by: John Harrison <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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