msm: mdss: skip bw vote for same bw requests

If the bandwidth requested is the same as the
one from the previous vote, skip the call to
bus driver in order to avoid an overhead
due unnecessary calls to bus driver.

Change-Id: I5e90f04a186d9d259e28c70783840294c86fcc98
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>