pan/midgard: Add an enum to describe the render targets

pan/midgard: Add an enum to describe the render targets

We are about to add a special ZS render target, let's add a enum so we
can easily know which render target we're dealing with.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3676>