drm/i915/tracepoints: Tidy request event class

At the moment only the global seqno is logged which is not set
until the request is ready for submission.

Add the per-contex seqno and the context hardware id which are
both interesting data points.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>