Implement drmGetCap() to query device/driver capabilities

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/xf86drm.h b/xf86drm.h
index 9b89f56..bf0d5df 100644
--- a/xf86drm.h
+++ b/xf86drm.h
@@ -542,6 +542,7 @@
 extern int           drmClose(int fd);
 extern drmVersionPtr drmGetVersion(int fd);
 extern drmVersionPtr drmGetLibVersion(int fd);
+extern int           drmGetCap(int fd, uint64_t capability, uint64_t *value);
 extern void          drmFreeVersion(drmVersionPtr);
 extern int           drmGetMagic(int fd, drm_magic_t * magic);
 extern char          *drmGetBusid(int fd);