The HW validation team came back from further testing with a slightly
changed constraint on the deviation between the DCO frequency and the
central frequency. Instead of +-4%, it's now +1%/-6%.

Unfortunately, the previous algorithm didn't quite cope with these new
constraints, the reason being that it wasn't thorough enough looking at
the possible divider candidates.

The new algorithm looks at all dividers, which is definitely a hammer
approach (we could reduce further the set of dividers to good ones as a
follow up, at the cost of a bit more complicated code). But, at least,
we can now satisfy the +1%/+6% rule for all the "Well known" HDMI
frequencies of my test set (373 entries).

On that subject, the new code is quite extensively tested in
intel-gpu-tools (tools/skl_compute_wrpll).

v2: Fix cycling between central frequencies and dividers (Paulo)
    Properly choose the minimal deviation between postive and negative
    candidates (Paulo).

    On the 373 test frequencies, v2 computes better dividers than v1 (ie
    more even dividers and lower deviation on average):

    v1: average deviation: 206.52
    v2: average deviation: 194.47
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>