drm/i915: Fix watermark calculation in self-refresh mode

For self-refresh mode WM calculation's "line time" should use
mode's htotal instead of hdisplay. "surface width" is the hdisplay
for display plane and 64 for cursor plane.

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>