drm/i915: Write all DDL registers in one go

We'll want to decouple the vlv/chv wm register reprogramming from any
single pipe. So let's just write all the DDL registers in one go. We
already write all the wm registers anyway since the bits are sprinkled
all over the place and so writing them for just a single pipe would have
been too messy anyway.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480354637-14209-14-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>