i965: Fix BRW_MEMZONE_LOW_4G heap size.

The STATE_BASE_ADDRESS "Size" fields can only hold 0xfffff in pages,
and 0xfffff * 4096 = 4294963200, which is 1 page shy of 4GB.

So we can't use the top page.

Fixes: 01058a55229 i965: Add virtual memory allocator infrastructure to brw_bufmgr.
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
1 file changed