drm/i915/tracepoints: Add hw_id to context tracepoints

It is useful to provide this info to match the one provided
in the request tracepoints.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170221091350.14605-1-tvrtko.ursulin@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>