emulators/ppsspp: drop Vulkan note after r487275
As the graphics team standardized on drm-kmod Vulkan no longer works
by default anywhere: AMD is blocked DRM_AUTH while Intel by USERPTR.
Workarounds exist but there's no point scattering them in every
Vulkan consumer.
emulators/ppsspp: advertise Vulkan support
- Vulkan lazy bindings are always enabled, built against bundled headers
- QT5 slave currently hardcodes GPUBackend::OPENGL