xf86drm: Reuse sysfs_uevent_get()

Recent patches for USB, platform and host1x bus support introduced the
sysfs_uevent_get() function that provides a generic way of parsing the
sysfs uevent file that is associated with each device in Linux.

Open-coded variants of this still exist in other places, so make those
reuse the new function to remove some code duplication.

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