drm/msm/dsi-staging: add min data rate to use phy regulator

Add minimum per lane data rate to control if regulator is required
to turn on. Below the minimum rate, phy regulator is not required
and can be turned off. With regulator off, phy power is sourced
from mx domain.

Change-Id: I1d717a848825c74f3b3f8a352bcba1e2b0ba88e6
Signed-off-by: Alan Kwong <akwong@codeaurora.org>