Stop undefining _ATOMIC_TYPE in Solaris/NetBSD section of xf86atomic.h

Since the atomic_set() macro references _ATOMIC_TYPE, undefining it
broke that macro, resulting in build errors of:
"intel_bufmgr_gem.c", line 775: undefined symbol: _ATOMIC_TYPE

Now that we're leaving the macro in the namespace, renamed it to start
with LIBDRM_ instead of _ to avoid polluting others namespace.

Regression introduced in 87fdd32c Add NetBSD atomic ops support.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-By: Thomas Klausner <wiz@NetBSD.org>