drm: remove drm_dev_set_unique

Not used by any drivers any more, the only use case in drm_dev_init()
can be inlined now.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230316082035.567520-2-christian.koenig@amd.com