panfrost: Set the MALI_WRITES_{Z,S} flags when needed

panfrost: Set the MALI_WRITES_{Z,S} flags when needed

In order to make Z/S writes from fragment shaders effective, we need
to set the MALI_WRITES_{Z,S} flags when the shader has a
FRAG_RESULT_{DEPTH,STENCIL} output variable.

Now that shaders can change the S value, we can expose the
STENCIL_EXPORT cap.

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/3697>