freedreno/a6xx: disallow UBWC for z24s8

freedreno/a6xx: disallow UBWC for z24s8

This is slightly annoying because it *mostly* works.. but we have some
issues to sort out about how to blit z24s8/x24s8/z24x8 with UBWC before
we can enable UBWC by default.  For now it is a step forward to at least
enable it for non-z/s while we figure out how to blit z24s8+UBWC.

(The basic issue is that pretending z24s8 is an equivalently sized rgba
format for the purpose of blitting falls apart when UBWC is in the
picture.)

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>