drm/i915: Add the support of memory self-refresh on Ironlake

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: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>