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