drm/i915: Warn if GPLL isn't used on vlv/chv

Our freq<->opcode conversions assume that GPLL is always used.
Apparently that should be the case always, but let's scream if we
ever encounter something different.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak S<deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>