x86/sev-es: Use GHCB accessor for setting the MMIO scratch buffer

Use ghcb_set_sw_scratch() to set the GHCB scratch field, which will also
set the corresponding bit in the GHCB valid_bitmap field to denote that
sw_scratch is actually valid.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Joerg Roedel <jroedel@suse.de>
Link: https://lkml.kernel.org/r/ba84deabdf44a7a880454fb351d189c6ad79d4ba.1601041106.git.thomas.lendacky@amd.com