drm/hisilicon: Adding a const declaration to an invariant construct

Some constructs cannot be changed after being assigned a value,
so add const declarations to invariant constructs.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1603955505-43942-1-git-send-email-tiantao6@hisilicon.com