drm/i915: make sure PC8 is enabled on suspend and disabled on resume v4

This matches the runtime suspend paths and allows the system to enter
the lowest power mode at freeze time.

v2: move disable_pc8 call to thaw_early (Imre)
    move enable_pc8 to freeze_late (Imre/Jesse)
v3: drop spurious hunk from _freeze now that we have freeze_late (Jesse)
v4: move back to suspend_late (Imre was right)

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