drm/hisilicon: Use simple encoder

The hibmc driver uses empty implementations for its encoders. Replace
the code with the generic simple encoder.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1609140936-57002-1-git-send-email-tiantao6@hisilicon.com