drm/rockchip: Use DRM_DEV_ERROR in vop

Since we can have multiple vops, use DRM_DEV_ERROR to
make logs easier to process.

Acked-by: Mark Yao <mark.yao@rockchip.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1471021254-2563-3-git-send-email-seanpaul@chromium.org