disp: msm: dsi: set default value of PPS CMD delay to zero

Fix the default value of post PPS command delay to zero, if
it is not specified as a panel device tree property.

Change-Id: I9aa972839d8be0620036595ac2514290cc6cf697
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>