drm/i915: use __wait_seqno for ring throttle

It turns out throttle had an almost identical  bit of code to do the
wait. Now we can call the new helper directly.  This is just a bonus,
and not needed for the overall series.

v2: remove irq_get/put which is now in __wait_seqno (Ben)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>