drm/i915: fix issue in display pipe setup on IGDNG

During pipe DPMS off, instead of busy waiting pipe off, insert
delays during wait and don't loop after enough tries which matches
spec requirement. Also try to match DPMS on path by disable FDI TX
PLL in DPMS off. Disable PF by writing PF_WIN_SZ which really trigger
the update.

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