drm/i915: Limit max VCO supported in CHV to 6.48GHz

As per the recommendation from PHY team, limit the max vco supported in CHV to 6.48 GHz

Signed-off-by: Vijay Purushothaman <vijay.a.purushothaman@linux.intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>