loader: reimplement loader_get_user_preferred_fd via libdrm

loader: reimplement loader_get_user_preferred_fd via libdrm

Currently not everyone has libudev and with follow-up patches we'll
completely remove the divergent codepaths.

Use the libdrm drm device API to construct the required ID_PATH_TAG-like
string, to preserve the current functionality for libudev users and
allow others to benefit from it as well.

v2: Drop ranty comments, pick the correct device
v3: \n -> \0 in PCI_ID_PATH_TAG_LENGTH comment (Axel).
v4: Use snprintf (Nicolai)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Axel Davy <axel.davy@ens.fr>