drm/client: Make client support optional

Only build client code if DRM_CLIENT has been selected. Automatially
do so if one of the default clients has been enabled. If client support
has been disabled, the helpers for client-related events are empty and
the regular client functions are not present.

Amdgpu has an internal DRM client, so it has to select DRM_CLIENT by
itself unconditionally.

v3:
- provide empty drm_client_debugfs_init() if DRM_CLIENT=n (kernel
  test robot)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Xinhui Pan <Xinhui.Pan@amd.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241014085740.582287-12-tzimmermann@suse.de