gallium: add z24s8_as_r8g8b8a8 format

gallium: add z24s8_as_r8g8b8a8 format

This maps to a special format that recent generations of adreno have,
for blitting z24s8.  Conceptually it is similar to doing Z and/or S
blits by pretending it is r8g8b8a8 (with appropriate writemask).  But
it differs when bandwidth compression is used, as z24 is a different
type from r8g8b8.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@gmail.com>