util/ralloc: Remove double zero'ing of rzalloc buffers

util/ralloc: Remove double zero'ing of rzalloc buffers

Juha-Pekka found this back in May 2015:
<1430915727-28677-1-git-send-email-juhapekka.heikkila@gmail.com>

From the discussion, obviously it would be preferable to make
ralloc_size no longer return zeroed memory, but Juha-Pekka found that
it would break Mesa.

In <56AF1C57.2030904@gmail.com>, Juha-Pekka mentioned that patches
exist to fix i965 when ralloc_size is fixed to not zero memory, but
the patches have not made their way to mesa-dev yet.

For now, let's stop doing the double zeroing of rzalloc buffers.

v2:
 * Move ralloc_size code to rzalloc_size, and add a comment as
   suggested by Ken.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>