drm/client: Rename drm_client_add() to drm_client_register()

This is done to stay consistent with our naming scheme of
_register() = others can start calling us from any thread.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190403125658.32389-1-noralf@tronnes.org


Patch-mainline: 20190403125658.32389-1-noralf@tronnes.org @ 03/04/19, 12:56 p.m.
[stalek@codeaurora.org: drop changes for drm_fb_helper.c as
the existing changes are not present.]

Change-Id: Ib33fe6d4e8a7ed056d5b97a2da0dbd61b23897f2
Signed-off-by: Shubham Talekar <stalek@codeaurora.org>