msm: mdss: Set default fps for dedicated WB

In targets with dedicated writeback paths, the FPS isn't
set properly for BW calculations for WB cases. Setting
the FPS to default frame rate, for such cases.

Change-Id: I83c545f3344ee5fc3d356f0e08d3c20453e22b31
Signed-off-by: Krishna Chaitanya Devarakonda <kdevarak@codeaurora.org>