drm/i915/skl: fetch, enable/disable pfit as needed v2

This moved around on SKL, so we need to make sure we read/write the
correct regs.

v2: fixup WIN_POS offsets (Paulo)
    zero out WIN_POS reg at disable time (Paulo)

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuougseek.org>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>