drm/i915: Uninterruptibly drain the timelines on unwedging

On wedging, we mark all executing requests as complete and all pending
requests completed as soon as they are ready. Before unwedging though we
wish to flush those pending requests prior to restoring default
execution, and so we must wait. Do so uninterruptibly as we do not provide
the EINTR gracefully back to userspace in this case but persists in
keeping the permanently wedged state without restarting the syscall.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208153708.20023-4-chris@chris-wilson.co.uk