panfrost: Constant stencil value tracking
If stencil is constant across the resource, then it can be treated as if it was cleared.
Improves performance in applications which create a stencil buffer but do not use it. Doing the same for depth would help some 2D applications, but creating a depth buffer and not using it isn't as common as for stencil buffers.
Draft because the issues mentioned in !16343 (merged) have not been fixed yet.