disp: msm: dsi: Fix porch calculation issue for constant fps

For constant fps feature, porch is calculated based on supported
clk rates. Currently, data type of local variables used for porch
calculation is u32 which leads to incorrect porch calculation for
higher clk rates. So, update the data type to u64.

Change-Id: I8eb04487d1dcce05989448c0b063e56752af412b
Signed-off-by: Lipsa Rout <lrout@codeaurora.org>
Signed-off-by: Ritesh Kumar <riteshk@codeaurora.org>