drm/writeback: remove pointless enable_signaling implementation

Just returning true here is a no-op, remove the optional callback.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240911085903.1496-4-christian.koenig@amd.com