drm/msm/dp: optimize link rate selection

Currently the maximum supported link rate is selected
for any given resolution. This may result in higher
system level power consumption.

Optimize this by checking the data clock of the preferred
resolution. Calculate the minimum link rate required for
this data clock and setup the link with that rate.

In case, user-space decides to go with a different resolution
which requires a different link rate, tear down the current
link session and setup the link again with the new rate.

CRs-Fixed: 2393144d
Change-Id: I4c2f7e540416328d8ec9c2be9de07ad0537c0b94
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>