drm/i915: Wake up the bottom-half if we steal their interrupt

Following on from the scenario Tvrtko envisioned to explain a hard-to-hit
race with multiple first waiters, we could also then race in the
__i915_request_irq_complete() and the bottom-half may miss the vital
irq-seqno barrier and so go to sleep not noticing their seqno is
complete.

v2: unlock, not double lock the rcu_read_lock.

Fixes: 3d5564e9 ("drm/i915: Only apply one barrier after...")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467805142-22219-2-git-send-email-chris@chris-wilson.co.uk