drm/i915: Spin until breadcrumb threads are complete

drm/i915: Spin until breadcrumb threads are complete

When we need to reset the global seqno on wraparound, we have to wait
until the current rbtrees are drained (or otherwise the next waiter will
be out of sequence). The current mechanism to kick and spin until
complete, may exit too early as it would break if the target thread was
currently running. Instead, we must wake up the threads, but keep
spinning until the trees have been deleted.

In order to appease Tvrtko, busy spin rather than yield().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161108143719.32215-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>