drm/i915: Rewrite vlv_find_best_dpll()

Rewrite vlv_find_best_dpll() to use intel_clock_t rather than
an army of local variables.

Also extract the code to calculate the derived values into
vlv_clock().

v2: Split up the earlier fixes, extract vlv_clock()
v3: Initialize best_clock

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>