r600g: Replace needless flush in texture upload.

Replace pipe->flush() with pipe->texture_barrier() in
the texture upload path for the staging texture.
This should be enough to get data out of the gpu
caches ready to be read for texture fetch.

src/gallium/drivers/r600/r600_texture.c

@@ -66,7 +66,7 @@ static void r600_copy_from_staging_texture(struct pipe_context *ctx, struct r600
 				  rtransfer->staging_texture,
 				  0, &sbox);
 
-	r600_flush(ctx, NULL, RADEON_FLUSH_ASYNC);
+	ctx->texture_barrier(ctx);
 }
 
 unsigned r600_texture_get_offset(struct r600_resource_texture *rtex,