Instead of swapping bo->mem just give old and new as parameters.

Also drop unused parameters and code.

v2: cleanup stale documentation as well.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210315191432.153826-3-christian.koenig@amd.com