Hi,
You will have to open the phone and check that the display assembly LCD screen and touchscreen cable connection to the systemboard is secure.
If it is, then as a DIY repair all you can do is to temporarily replace the display assembly and check if the problem is still there or not. If it is still there then it is in the systemboard
Here’s a [https://www.youtube.com/watch?v=lRj_F7b4JvE|video] that shows how to replace the display assembly in your phone.
Replacement display assemblies are available online. Just search for ''Blackberry PRIV LCD display touchscreen assembly'' to find suppliers that suit you