drm/i915/chv: use vlv_PLL_is_optimal in chv_find_best_dpll

Prepare chv_find_best_dpll to be used for BXT too, where we want to
consider the error between target and calculated frequency too when
choosing a better PLL configuration.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>