drm/i915: Restrict hsw_dp_set_ddi_pll_sel() to HSW/BDW

Future platform will use config->ddi_pll_sel in a different way.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>