drm: drm_printer: add __printf validation

drm_printf does not currently use the compiler to verify
format and arguments.  Make it do so.

Miscellanea:

o Add appropriate #include files for __printf and struct va_format
o Convert dev_printk to dev_info

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/133858f214e9b90f92bb8eb44c6b1dc04429933d.1487201526.git.joe@perches.com
diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
index 02a107d..74c466a 100644
--- a/drivers/gpu/drm/drm_print.c
+++ b/drivers/gpu/drm/drm_print.c
@@ -36,7 +36,7 @@ EXPORT_SYMBOL(__drm_printfn_seq_file);
 
 void __drm_printfn_info(struct drm_printer *p, struct va_format *vaf)
 {
-	dev_printk(KERN_INFO, p->arg, "[" DRM_NAME "] %pV", vaf);
+	dev_info(p->arg, "[" DRM_NAME "] %pV", vaf);
 }
 EXPORT_SYMBOL(__drm_printfn_info);