Use drm_err instead of DRM_ERROR in hibmc_drm_vdac

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1597829014-39942-3-git-send-email-tiantao6@hisilicon.com