kms: Fix compilation error when libdrm < 2.4.68

kms: Fix compilation error when libdrm < 2.4.68

DRM_RDWR was not defined until libdrm 2.4.68.  However,
in configure.ac we only require libdrm >= 2.4.55.

Seems silly to to bump minimum libdrm version for a simple
define.  Thus, define DRM_RDWR if it's not defined.

This fixes compilation error introduced in:

commit 922031b0f98bf5c7b01f4431d1c08617f65865f7
Author: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Date:   Tue Sep 12 12:07:13 2017 -0400

    kms: Export DMABuf from Dumb buffer when possible

    https://bugzilla.gnome.org/show_bug.cgi?id=787593

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>