msm: mdss: fix incorrect assignment issue in bw calculation

There is no need to assign the output of do_div macro as it
updates the first argument with the result. Fix the same to
address the issue with bandwidth calculation.

Change-Id: I5aca09da89ecc20ca8c998f3169c852c18194ba6
Signed-off-by: Ramakant Singh <ramaka@codeaurora.org>