drm/i915: Fix fifo size for self-refresh watermark on 965G

The total self-refresh fifo entry size for display plane is 512
instead of 128 for 965G. Also fix WM value mask for 965G.

About 1.0W power can be saved on one T61 laptop after the self-refresh
watermark is configured correctly.

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>