llvmpipe: turn depth clears into full depth/stencil clears for d24x8 formats

llvmpipe: turn depth clears into full depth/stencil clears for d24x8 formats

If we have a d24x8 format, there is no stencil. Therefore, we can always
clear these bits too, which means this will be some kind of memset rather
than read-modify-write.
This is good for some 7% increase or so in gears with huge window size -
seems to have a bigger effect if things aren't in caches. Of course, any
real app won't spend nearly as much time comparatively in clearing
depth buffer in the first place, so the speedup will be much lower.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>