Correctly set DRM_MAX_MINOR for all platforms.

DRM_MAJOR is platform specific, but not used outside of xf86drm.c
that I can find.
diff --git a/xf86drm.h b/xf86drm.h
index 9944d32..496d95d 100644
--- a/xf86drm.h
+++ b/xf86drm.h
@@ -39,6 +39,10 @@
 #include <stdint.h>
 #include <drm.h>
 
+#ifndef DRM_MAX_MINOR
+#define DRM_MAX_MINOR   16
+#endif
+
 #if defined(__linux__)
 
 #define DRM_IOCTL_NR(n)		_IOC_NR(n)
@@ -47,8 +51,6 @@
 #define DRM_IOC_WRITE		_IOC_WRITE
 #define DRM_IOC_READWRITE	_IOC_READ|_IOC_WRITE
 #define DRM_IOC(dir, group, nr, size) _IOC(dir, group, nr, size)
-#define DRM_MAJOR       226
-#define DRM_MAX_MINOR   15
 
 #else /* One of the *BSDs */