drm/client: Move public client header to clients/ subdirectory

Move the public header file drm_client_setup.h to the clients/
subdirectory and update all drivers. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241108154600.126162-3-tzimmermann@suse.de