msm: mdss: add support to configure bus scale vectors from dt

Add support to configure the bus scale vectors from
mdss device tree.
Use Levels instead of freq values for proper abstraction.

Change-Id: I28d9f77a801aa6970ba1280cc1b1b2c5cc7eaf4a
Signed-off-by: Kalyan Thota <kalyant@codeaurora.org>