drm/i915: Apply self-refresh watermark calculation for cursor plane

In SR mode cursor plane watermark calculation uses same formula
like display plane. This one fixes the case for 965G and G45.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>