panfrost: Use ctx->wallpaper_batch in panfrost_blit_wallpaper()

panfrost: Use ctx->wallpaper_batch in panfrost_blit_wallpaper()

We'll soon be able to flush a batch that's not currently bound to the
context, which means ctx->pipe_framebuffer will not necessarily be the
FBO targeted by the wallpaper draw. Let's prepare for this case and
use ctx->wallpaper_batch in panfrost_blit_wallpaper().

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>