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: 01058a55 i965: Add virtual memory allocator infrastructure to brw_bufmgr. Reviewed-by:Jason Ekstrand <jason@jlekstrand.net> (cherry picked from commit 15f134c6)