drm: i2c: tda998x: constify i2c_device_id

i2c_device_id are not supposed to change at runtime. All functions
working with i2c_device_id provided by <linux/i2c.h> work with
const i2c_device_id. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1503167300-17334-4-git-send-email-arvind.yadav.cs@gmail.com
Link: https://patchwork.freedesktop.org/patch/msgid/60db0ba6ddce08444299533e97c9adec3b3ef1bb.1503311145.git.arvind.yadav.cs@gmail.com