drm/i915: Update the dispatch tracepoint to use params->request

Updated a couple of trace points to use the now cached request pointer rather
than extracting it from the ring.

For: VIZ-5115
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Tomas Elf <tomas.elf@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>