drm/msm/dp: use correct clock after rate down shift

If link training fails for a given link rate, we go
down to a lower link rate. In case link training is
successful on the lower link rate, use that link rate
to validate the display modes that can be supported
on that rate. This is needed to avoid selecting a
display mode that can't be used for a lower link rate.

Change-Id: Id02de88db508d034d16749bce826b00111753cd9
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>