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.

v2: Don't do anything if a fragment job wouldn't be needed otherwise.