drm/i915/skl: Use a LRI for WaDisableDgMirrorFixInHalfSliceChicken5

I have no idea how that crept in, but we need to do the write from the
ring and this is a masked register. Two fixes in 1!

Cc: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>