r600g: use a buffer in GTT as intermediate on texture up and downloads

r600g: use a buffer in GTT as intermediate on texture up and downloads

Generalize the existing tiled_buffer path in texture transfers for use
in some non-tiled up and downloads.

Use a staging buffer, which the winsys will restrict to GTT memory.

GTT buffers have the major advantage when they are mapped, they are
cachable, which is a very nice property for downloads, usually the CPU
will want to do look at the data it downloaded.