msm_fb: display: add mdp clk rate calculation for low vbp

For the panels that have low v_back_porch, if h/w FIR scaling is used,
during vbp time, mdp clk should be fast enough to output a line from 4
input lines. MDP rate equation for very small scaling, e.g. 1 or 2
pixels' upscaling or downscaling cannot cover this case, where
underruns are seen on those panels.

Change-Id: I4f35f98c2c1bc4ef04dfee193427bd3ff674945d
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>