drm/i915: Perform Sandybridge BSD tail write under the forcewake

Since we have a sequence of register reads and writes, we can reduce the
latency of starting the BSD ring by performing all the mmio operations
under the same forcewake wakeref.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467297225-21379-62-git-send-email-chris@chris-wilson.co.uk