mesa: improve texstore for 8/24 formats and add texstore for S8.

mesa: improve texstore for 8/24 formats and add texstore for S8.

this improves mesa texstore for 8/24 so it can create S24X8/X24S8 variants
by keeping the depth bits static.

it also adds a texstore for S8 so we can write out an S8 texture to use
in the sampler for accel draw pixels to save memory bw.

The logic seems sound here, I've worked it out a few times on paper, though
it would be good to have some review.

Signed-off-by: Dave Airlie <airlied@redhat.com>