drm/i915: Make request conflict tracking understand parallel submits

If an object in the excl or shared slot is a composite fence from a
parallel submit and the current request in the conflict tracking is from
the same parallel context there is no need to enforce ordering as the
ordering is already implicit. Make the request conflict tracking
understand this by comparing a parallel submit's parent context and
skipping conflict insertion if the values match.

v2:
 (John Harrison)
  - Reword commit message

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
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/20211014172005.27155-23-matthew.brost@intel.com