drm/i915: Unifiy GT powersave suspend logic

Jesse's patch to only quiescent our rps work and Imre's fix to address
a race with runtime pm and the forcewake reference held by the used
diverging means to address the same bug: Jesse's patch uses
flush_delayed_work while (since we want to make sure rps is set up)
while Imre's used a cancel+manuel refcount adjustment.

Unify them again by simply reusing intel_suspend_gt_powersave in
intel_disable_gt_powersave.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>