x86, amd: Cleanup init_amd

Distribute family-specific code to corresponding functions.

Also,

* move the direct mapping splitting around the TSEG SMM area to
bsp_init_amd().

* kill ancient comment about what we should do for K5.

* merge amd_k7_smp_check() into its only caller init_amd_k7 and drop
cpu_has_mp macro.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1403609105-8332-3-git-send-email-bp@alien8.de
Signed-off-by: H. Peter Anvin <hpa@zytor.com>