xf86drm: Add USB support

Allow DRM/KMS devices hosted on USB to be detected by the drmDevice
infrastructure.

v4:
- continue on error to process USB devices

v3:
- guard Linux-specific sysfs parsing code with #ifdef __linux__

v2:
- make sysfs_uevent_get() more flexible using a format string

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>