When storing depth- or stencil-only texture data that has been packed into
a depth/stencil texture, the tex store gets PIPE_MAP_READ added onto it
since the other channel will get ORed into the incoming data, but
sometimes we know that the other component is undefined because the whole
texture is either fresh or just invalidated.

Cleans up a confusing extra blit in a dEQP case I've been debugging, and
should be less work for dEQP CI.

Part-of: <mesa/mesa!11452>