xf86drm: remove makedev() hack/workaround

Back when this was introduced commit 569da5a4(Merged glxmisc-3-0-0)
sys/sysmacros.h was used instead of the respecive headers (as per the
manual).

We've been handling it correctly for a little while now - in Linux, BSD
and Solaris. Thus we can drop this workaround.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>