Fix one warning (v2)

xf86drm.c:356:2: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
  group = (serv_group >= 0) ? serv_group : DRM_DEV_GID;
  ^

v2: do 'int' cast to fix the warning

Signed-off-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>