Mark functions printf-like where possible

These functions all take a format string and either a list of variable
arguments or a va_list. Use the new DRM_PRINTFLIKE macro to tell the
compiler about it so that the arguments can be checked against the
format string.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Thierry Reding <treding@nvidia.com>
diff --git a/xf86drm.h b/xf86drm.h
index 5e170f8..c024cc4 100644
--- a/xf86drm.h
+++ b/xf86drm.h
@@ -704,7 +704,7 @@
 
 extern int drmOpenOnce(void *unused, const char *BusID, int *newlyopened);
 extern void drmCloseOnce(int fd);
-extern void drmMsg(const char *format, ...);
+extern void drmMsg(const char *format, ...) DRM_PRINTFLIKE(1, 2);
 
 extern int drmSetMaster(int fd);
 extern int drmDropMaster(int fd);