Revert "drm/i915: Drop ring->lazy_request"

With multiple rings generating requests independently, the outstanding
requests must also be track independently.

Reported-by: Wang Jinjin <jinjin.wang@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30380
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>