Update the self-refresh watermark for display plane/cursor and enable
the memory self-refresh on Ironlake. The watermark is also updated for
the active display plane.

More than 1W idle power is saved on one Ironlake laptop after enabling
memory self-refresh.
Signed-off-by: NZhao Yakui <yakui.zhao@intel.com>
Signed-off-by: NZhenyu Wang <zhenyuw@linux.intel.com>