x86/sev: Share implementation of MSR-based page state change

Both the decompressor and the SEV startup code implement the exact same
sequence for invoking the MSR based communication protocol to effectuate
a page state change.

Before tweaking the internal APIs used in both versions, merge them and
share them so those tweaks are only needed in a single place.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250828102202.1849035-31-ardb+git@google.com