vulkan/device_select: fix multiple DRI_PRIME bugs and add optional debug logging

Fixes #7498 (closed)

• DRI_PRIME=any_invalid_value or 0 now does nothing instead of selecting the last GPU (which is usually llvmpipe)
• DRI_PRIME=pci-0000_06_00_0 now selects the device with that PCI bus address instead of selecting the last GPU
• DRI_PRIME=1 now picks the first GPU if the active GPU is the third or later GPU, instead of doing nothing
• Adds MESA_VK_DEVICE_SELECT_DEBUG which activates debug logging of why a selection was made