arm/dt: lenok: Fix the flickering on display

This issue happens when POLED PANEL gate driving voltage is unstable.
Adjust the panel driving voltage level to avoid this issue.

Change VGL to -7.5V from -6.5V. It doesn't affect on power.

Bug: 18087424
Change-Id: I985531ef3d2424306ca2eca2d571174a80491cd4
Signed-off-by: sol.yu <sol.yu@lge.com>
Signed-off-by: Devin Kim <dojip.kim@lge.com>