drm/i915: factor out vlv_PLL_is_optimal

Factor out the logic to decide whether the newly calculated dividers are
better than the best found so far. Do this for clarity and to prepare
for the upcoming BXT helper needing the same.

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>