xf86drm: Factor out drmDeviceAlloc()

Subsequent patches will add support for other bus types to drmDevice and
they will duplicate a lot of the code to allocate a drmDevice. Factor
out the common code so it can be reused.

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