radv: store the list of attachments for every subpass

radv: store the list of attachments for every subpass

This reworks how the depth stencil attachment is used for
simplicity. This also introduces radv_render_pass_compile()
helper that will be used for further optimizations.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>