xf86drm: mark DRM_MAX_MINOR as deprecated

With the work in [1], libdrm users should no longer rely on the
minor numbering scheme we've used so far. Instead, they should use
drmGetDevices2().

[1]: https://lore.kernel.org/lkml/20230724211428.3831636-1-michal.winiarski@intel.com/

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

xf86drm.h

@@ -44,7 +44,7 @@ extern "C" {
 #endif
 
 #ifndef DRM_MAX_MINOR
-#define DRM_MAX_MINOR   64
+#define DRM_MAX_MINOR   64 /* deprecated */
 #endif
 
 #if defined(__linux__)