x86: clean up init_amd()

1. move out calling of check_enable_amd_mmconf_dmi out of setup_64.c
   put it into init_amd(), so don't need to make extra dmi check for
   system with other cpus.
2. 15 --> 0xf

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