libdrm: Use userspace compatible type in fourcc_mod_code macro

__u64 should be used instead of u64.

Kernel headers originally pulled in:

commit 8983fe54
Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Date:   Mon Aug 3 10:48:03 2015 +0100

    libdrm: Add framebuffer modifiers uapi

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Rob Clark <robdclark@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rob Clark <robclark@freedesktop.org>