xf86drm: dedupe `#define`s

Adapted from a local patch carried by DragonFlyBSD:
https://github.com/DragonFlyBSD/DPorts/blob/bc056f88f7e4d468d8c9751f831a47b5ae1326e3/graphics/libdrm/files/patch-xf86drm.h



Patch is sadly uncredited (a bot authored the commit), so I can't credit
the author here either.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>