msm: mdss: change calculation reference to default frame rate

In the current implementation for dynamic fps in vfp method, while
calculating the porch values current fps is taken as reference which
may lead to precision loss and clock calculations go wrong. Use the
default fps as reference for any change in fps.

Change-Id: I9a36c33c9824c95abc4925a324f9cd2f47456e6a
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>