pvr: Handle barrier load and store flags.

pvr: Handle barrier load and store flags.

This commit adds handling for {s,z}loaden and {s,z}storeen to
control loading from and storing to the stencil and depth buffer.

This commit also addressed the FIXMEs around barrier_{load,store}
which control the {s,z}{load,store}en.

Signed-off-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20487>