llvmpipe: Cap to 2 GB on 32 bits

llvmpipe: Cap to 2 GB on 32 bits

On 32 bits system, application memory is quite limited.
llvmpipe uses application memory. To help prevent memory
exhaustion, limit reported memory availability to 2GB.

Some gallium nine apps do check reported memory by allocating
resources until memory is full. Gallium nine refuses allocations
when 80% of the reported memory limit is used. This change
helps some apps to start.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>