add platform-probe support

Signed-off-by: Rob Clark <robdclark@gmail.com>
add support for server managed fd's

This enables xf86-video-freedreno to work with a non-root xserver.  (Ie.
systemd-logind or similar opens the device and becomes master, and then
passes the fd to xserver which passes it to driver).

Signed-off-by: Rob Clark <robdclark@gmail.com>