drm: Avoid calling dev_printk(.dev = NULL)

Since dev_printk likes to print "(NULL device *):" when passed in a NULL
pointer, we have to manually call printk() ourselves.

Fixes: c4e68a58 ("drm: Introduce DRM_DEV_* log messages")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Eric Engestrom <eric.engestrom@imgtec.com>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20160819073750.16610-1-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_drv.c

@@ -79,8 +79,11 @@ void drm_dev_printk(const struct device *dev, const char *level,
 	vaf.fmt = format;
 	vaf.va = &args;
 
-	dev_printk(level, dev, DRM_PRINTK_FMT, function_name, prefix,
-		   &vaf);
+	if (dev)
+		dev_printk(level, dev, DRM_PRINTK_FMT, function_name, prefix,
+			   &vaf);
+	else
+		printk("%s" DRM_PRINTK_FMT, level, function_name, prefix, &vaf);
 
 	va_end(args);
 }