msm: mdss: reduce PPP bw vote for 1080p video on 8909

On 8909 1080p video only up to 30fps is supported.
Take this into account while calculating bandwidth
requirement as currently we are assuming mipi frame
rate for all the layers.

Change-Id: I22b9603d31a119c9ee073369e31e39bae0760c4b
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>