drm/i915: Update ring->flush() to take a requests structure

Updated the various ring->flush() functions to take a request instead of a ring.
Also updated the tracer to include the request id.

For: VIZ-5115
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Tomas Elf <tomas.elf@intel.com>
[danvet: Rebase since I didn't merge the addition of req->uniq.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>