sna/gen6: Prefer the render ring for copies

Slower for fills, but on the current stack faster for copies, both large
and small. Hopefully, when we write some good shaders for SNB, we will
not only improve performance for copies but also make fills faster on
the render ring than the blt?

As the BLT copy routine is GPU bound for copywinpix10, and the RENDER
copy routine is CPU bound and faster, I believe that we have reached the
potential of the BLT ring and not yet saturated the GPU using the render
copy.

Note that we still do not casually switch rings, so the actual routine
chosen will still be selected by the preceeding operations, so is
unlikely to have any effect in practice during, for example, cairo-traces.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/gen6_render.c

@@ -3167,7 +3167,7 @@ static inline bool prefer_blt_copy(struct sna *sna,
 				   PixmapPtr src, struct kgem_bo *src_bo,
 				   PixmapPtr dst, struct kgem_bo *dst_bo)
 {
-	return (sna->kgem.ring != KGEM_RENDER ||
+	return (sna->kgem.ring == KGEM_BLT ||
 		prefer_blt_bo(sna, src, src_bo) ||
 		prefer_blt_bo(sna, dst, dst_bo));
 }