x86: don't need to go to chunksize to 4G

change back chunksize max to 2g
   otherwise will get strange layout in 2G ram system like
     0 - 4g WB, 2040M - 2048M UC, 2048M -  4G NC
   instead of
     0 - 2g WB, 2040M - 2048M UC

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>