ARM: dts: msm: decrease porch to fix screen flicker on sdm429

Decrease porch value to fix DUT screen flicker during long time
stability test on hx8399c hd+ panel.

Change-Id: I93f88514cfea16d74db8f071f51799016a4e92cb
Signed-off-by: Wenjun Zhang <wjzhan@codeaurora.org>