drm/i915: Calculate cursor watermark under non-SR state for Ironlake

The hardware team suggest that the "large buffer" method should be
used to calculate the cursor watermark under non-SR state as well,
which is to avoid the flicker when FBC is enabled on Ironlake.

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>