x86/sev-es: Optimize __sev_es_ist_enter() for better readability

Reorganize the code and improve the comments to make the function more
readable and easier to understand.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210303141716.29223-4-joro@8bytes.org