msm: mdss: correct the mdp clock calculation logic

Currently scale values are incorrectly calculated during
mdp clock calculation for ppp engine. Correct this logic
to avoid incorrect mdp clock voting resulting in
performance issues.

Change-Id: I8374039f17e70a6d0eaffdaac3e49c470984a0ad
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>