The GPU (at least a3xx, but I think also a2xx) can render directly to
memory, bypassing tiling.  Although it can't do this if blend, depth,
and a few other features of the pipeline are enabled.  This direct
memory mode can be faster for some sorts of operations, such as simple
blits.  In particular, this significantly speeds up XA by avoiding to
pull the entire dest pixmap into GMEM, render tiles, and write it all
back out again.  This should also speed up resource copy-region and
blit.

Signed-off-by: Rob Clark <robclark@freedesktop.org>