st/xa: Cache render target surface

st/xa: Cache render target surface

Otherwise it will trick the gallium driver into thinking that the render
target has actually changed (due to different pipe_surface pointing to
same underlying pipe_resource).  This is really badness for tiling GPUs
like adreno.

This also appears to fix a rendering error with Motif on vmwgfx.
Why that is is still under investigation.

Based on an idea by Rob Clark.

Cc: "10.0 10.1" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Rob Clark <robclark@freedesktop.org>