msm: mdss: calculate rotator bw based on source fps

If the source FPS for the rotator is set, store it and
use it for calculating BW required by rotator.

Change-Id: I35801528930897447a039c4a4c950048e99d16d9
Signed-off-by: Krishna Chaitanya Devarakonda <kdevarak@codeaurora.org>