I tend to debug like a caveman so I don't know if I'm the right person for that - in this case, I just went through config options (make menuconfig) and turned on CONFIG_DRM_I915_DEBUG and a bunch of associated things, then built and booted and played around. Got nothing useful out of that but I didn't spend a huge amount of time diving into the driver.

In this case it's pretty different to regular kernel debug because I'm not debugging kernel code itself, I'm trying to glean more information out of hardware that I have no knowledge of, it's not as simple as "there's a bug on line X and I need to find out why this pointer is invalid" etc.