msm: mdss: Fix qseed3 clk calculation overflow

Use u64 variables to store intermediate results while calculating
mdp clock for qseed3 scaling scenarios, using u32, results in overflow
and incorrect mdp clock setting.

Change-Id: I4fbc9aa1f30d36d35a9ad181185761e697cbbef7
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>