platforms/drm: disable explicit modifiers by default

There are some unresolved issues where driving outputs can fail because of bandwidth constraints. These don't appear to happen with implicit modifiers, or at least they don't happen as often.

To use modifiers by default again we'll likely have to test lots of modifier combinations in DrmGpu::findWorkingCombination, not just connector+crtc combinations. Not a very difficult thing to do per se but will get annoying and impossibly slow in some cases, and definitely does not need to be backported.