drm/i915: Assert that HSW/BDW LCPLL is using the non-SSC reference

Only the non-SSC reference is truly supported for the LCPLL. Assert
that it is indeed selected.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190604200933.29417-4-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>