xf86drm: parse the separate sysfs files for vendor... info

Up-to recently (patch should land in 4.10) the kernel did not expose the
PCI device revision field as a separate sysfs file.

Thus one needed too parse the config file to retrieve it. This in
itself wakes up the device, which in some cases can be quite slow.

To avoid that, just check for the separate files and fall-back to the
original if kernel is not new enough.

v3: rework alongside drmGetDevice[s]2

Cc: Michel Dänzer <michel@daenzer.net>
Cc: Nicolai Hähnle <nhaehnle@gmail.com>
Cc: Mauro Santos <registo.mailling@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98502


Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>