msm: mdss: Move msm register bus setup outside of postproc

The msm register bus is currently configured by postprocessing. If the
mdss driver wants to modify the msm register bus bandwidth at any point
in the future, then the bus should be configured in a more central
location of the mdss driver.

Change-Id: I4ae65f4448f0b2be1371446d08bd4ab55c830d19
Signed-off-by: Benet Clark <benetc@codeaurora.org>