gallium/u_transfer_helper: Don't leak a reference to the resource.

We pipe_resource_reference when handling transfers in map, we need to
do a corresponding unreference in unmap.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
10 jobs for !1199 with transfer-leaks in 8 minutes and 42 seconds (queued for 6 seconds)
merge request