msm: mdss: add support for hw_rt bus client for mdss

Few targets expect a minimum vote on the bus during
SMMU and TZ operations. Add a new bus client in mdss
driver which will be used to request the additional
vote.

Change-Id: I0b84479bf892def42c0b59a684a850d8d5c01257
Signed-off-by: Kalyan Thota <kalyant@codeaurora.org>