drm/i915/bdw: get the correct LCPLL frequency on Broadwell

v2: Rebased onto Paulo's MHz->kHz change.

v3: Rebased on top of the Haswell pc8+ adjustements.

v4: Use the exact 337.5MHz clock, should have been done as part of v2.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v1)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>