loader: Avoid enumerating drm devices just to get and fd's PCI ID.
Cuts 1/3 of the runtime of the VA-API unit tests (which do a separate pipe-loader init per test) on radeonsi on my system by not faffing around in sysfs so much.
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Cuts 1/3 of the runtime of the VA-API unit tests (which do a separate pipe-loader init per test) on radeonsi on my system by not faffing around in sysfs so much.