x86/sev: Don't use cc_platform_has() for early SEV-SNP calls

When running identity-mapped and depending on the kernel configuration,
it is possible that the compiler uses jump tables when generating code
for cc_platform_has().

This causes a boot failure because the jump table uses un-mapped kernel
virtual addresses, not identity-mapped addresses. This has been seen
with CONFIG_RETPOLINE=n.

Similar to sme_encrypt_kernel(), use an open-coded direct check for the
status of SNP rather than trying to eliminate the jump table. This
preserves any code optimization in cc_platform_has() that can be useful
post boot. It also limits the changes to SEV-specific files so that
future compiler features won't necessarily require possible build changes
just because they are not compatible with running identity-mapped.

  [ bp: Massage commit message. ]

Fixes: 5e5ccff6

 ("x86/sev: Add helper for validating pages in early enc attribute changes")
Reported-by: Sean Christopherson <seanjc@google.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: <stable@vger.kernel.org> # 5.19.x
Link: https://lore.kernel.org/all/YqfabnTRxFSM+LoX@google.com/