u_tile: Skip the packed temporary and just store tiles directly.

We were generating a packed copy and then memcpying it, but we can just pack directly to the destination. Change on glmark2 -b build:use-vbo=true is modest: 1.06328% +/- 0.994771% (n=84) but does remove the function that was .6% of CPU time.

I'm not doing the equivalent "get" path at this time because softpipe's texture cache has some clipping issues that get revealed.