drm/i915: Only include active engines in the capture state

Skip printing out idle engines that did not contribute to the GPU hang.
As the number of engines gets ever larger, we have increasing noise in
the error state where typically there is only one guilty request on one
engine that we need to inspect.

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/20190808144511.32269-1-chris@chris-wilson.co.uk