drm/i915/tgl: Fix dkl link training

drm/i915/tgl: Fix dkl link training

Link training is failling when running link at 2.7GHz and 1.62GHz and
following BSpec pll algorithm.

Comparing the values calculated and the ones from the reference table
it looks like MG_CLKTOP2_CORECLKCTL1_A_DIVRATIO should not always set
to 5. For DP ports ICL mg pll algorithm sets it to 10 or 5 based on
div2 value, that matches with dkl hardcoded table.

So implementing this way as it proved to work in HW and leaving a
comment so we know why it do not match BSpec.

v4:
Using the same is_dp check as ICL, need testing on HDMI over tc port

Issue reported on BSpec 49204.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190926210659.56317-3-jose.souza@intel.com