Make sure that the RPS bottom-half is flushed before we set the idle
frequency when we decide the GPU is idle. This should prevent any races
with the bottom-half and setting the idle frequency, and ensures that
the bottom-half is bounded by the GPU's rpm reference taken for when it
is active (i.e. between gen6_rps_busy() and gen6_rps_idle()).

v2: Avoid recursively using the i915->wq - RPS does not touch the
struct_mutex so has no place being on the ordered i915->wq.
v3: Enable/disable interrupts for RPS busy/idle in order to prevent
further HW access from RPS outside of the wakeref.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
References: https://bugs.freedesktop.org/show_bug.cgi?id=89728Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467616119-4093-6-git-send-email-chris@chris-wilson.co.uk