vk/formats: Remove the cpp=0 stencil hack

vk/formats: Remove the cpp=0 stencil hack

The format table defined cpp = 0 for stencil-only formats. The real cpp
is 1.

When code begins to lie, especially about stencil buffers, code becomes
increasingly fragile as time progresses, and the damage becomes
increasingly hard to undo. (For precedent, see the painful history of
stencil buffer cpp in the git log for gen6 and gen7 in the i965 driver).
Let's undo the stencil buffer cpp lie now to avoid future pain.

In the format table, set cpp = 1 for VK_FORMAT_S8; replace checks for
cpp == 0; and delete all comments about the hack.