drm/msm/dp: fix link rate down shifting

Start the link training with maximum supported link rate. If
link training fails at that rate, calculate the minimum
required link rate for a given pixel rate. Switch to the next
lower link rate if that is still higher than the minimum
required rate and then retry the link training.
This is needed to retry the link training at different link
rates in case it fails at current set link rate.

CRs-Fixed: 2072336
Change-Id: I792115e3564ed919ce0200699c3f564df0aba8aa
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>