msm: mdss: Compensate for low VBP in MDP clock calculations

MDP latency is 4 lines with CAF and when the panel's vertical
Back porch value is lower than 4, the MDP clk rate needs to
be increased by 25% to compensate for the latency.

Change-Id: I732de52b4f5387d5c7992e893ea7cad5c6951811
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>