drm/msm/dp: downshift the link rate during link training logic

For the link training related tests during Link CTS, there can
be a need to downshift the link rate level lower than the
minimum link rate needed to support a given display resolution.
This is needed both in the HPD high and HPD IRQ path. The Link
Training flow chart in the spec doesn't insist on limiting the
downshift to the minimum required link rate for the resolution.
Hence, allow the complete downshift of the link rate till 1.62
Ghz if needed.

Change-Id: I91135c315c1b6c252cc8751da429a15d8b92ec9f
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>