x86/sev: Replace occurrences of sev_active() with cc_platform_has()

Replace uses of sev_active() with the more generic cc_platform_has()
using CC_ATTR_GUEST_MEM_ENCRYPT. If future support is added for other
memory encryption technologies, the use of CC_ATTR_GUEST_MEM_ENCRYPT
can be updated, as required.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210928191009.32551-7-bp@alien8.de