drm/i915: Pass around dev_priv for ddi buffer programming

Make the ddi buffer programming code a bit more neat by passing
around dev_priv instead of dev.

v2: Resolve conflicts due to KBL

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)