drm/i915/gt: Show the cumulative context runtime in engine debug

As we have the total runtime known to us, show it when dumping the
engine state for debug.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200218162150.1300405-2-chris@chris-wilson.co.uk