drm/i915/bdw: Use The GT mailbox for IPS enable/disable

v2: Squash in fixup from Ben to synchronize the GT mailbox commands.

CC: Art Runyan <arthur.j.runyan@intel.com>
Reviewed-by: Art Runyan <arthur.j.runyan@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>