i965/blorp: Explain why Z24 can't use a sensible format.

i965/blorp: Explain why Z24 can't use a sensible format.

We could conceivably use BRW_SURFACEFORMAT_R24_UNORM_X8_TYPELESS for
Z24 source images, allowing conversions from Z24 to either Z16 or Z32F.

Unfortunately, we can't use it for destination images since it isn't
supported as a render target.

Using different formats for sources or destinations would be painful,
so for now, punt.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>