drm/i915: Remove superfluous powersave work flushing

Instead of flushing the outstanding enabling, remember the requested
frequency to apply when the powersave work runs.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468397438-21226-6-git-send-email-chris@chris-wilson.co.uk