Find PipeWire only if needed (by BUILD_TESTING); drop Libdrm dependency
I want to be really careful there but at least on my system, this built fine without Libdrm and I don't see where else PipeWire is used.
- Drop unused Libdrm dependency
- Put PipeWire check behind BUILD_TESTING
- Only autotests link to PipeWire, so only bail out there if not found