nvc0: fix up image support for allowing multiple samples

nvc0: fix up image support for allowing multiple samples

Basically we just have to scale up the coordinates and then add the
relevant sample offset. The code to handle this was already largely
present from Christoph's earlier attempts to pipe images through back in
the dark ages, this just hooks it all up.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>