drm/i915/gt: Show the culmative runtime as part of the engine info

Since we always enable the busy-stats, the culmulative runtime should be
accurate, and might be useful for diagnosing issues with the engine.

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/20200619191053.9654-1-chris@chris-wilson.co.uk