drm/i915: Move retire-requests into i915_gem_wait_for_idle()

drm/i915: Move retire-requests into i915_gem_wait_for_idle()

As we now distinguish everywhere that can call
i915_gem_retire_requests() following a successful wait_for_idle, we can
remove the duplication by moving that call into i915_gem_wait_for_idle()
itself.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170330145041.9005-3-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>