drm/core: clean up references to drm_dev_unref()

This is a continuation of a previous commit ("drm: introduce drm_dev_{get/put}
functions") to replace all references to drm_dev_unref() in drm core files with
drm_dev_put().

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170926170400.GA7671@aishwarya