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