drm/i915/skl: Prefer even dividers for SKL DPLLs

Currently, if an odd divider improves the deviation (minimizes it), we
take that divider. The recommendation is to prefer even dividers.

v2: Move the check at the right place after having inverted the two for
    loops in the previous patch.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>