xf86drm: add drm{Get,Free}Device

Similar interface to the *Devices() ones but they obtain/free the
information of the opened device (as given by its fd).

Note there is a fair bit of duplication between the two Get functions,
and anyone interested is more than welcome to consolidate it.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>