x86/hyperv: Set MTRR state when running as SEV-SNP Hyper-V guest

x86/hyperv: Set MTRR state when running as SEV-SNP Hyper-V guest

In order to avoid mappings using the UC- cache attribute, set the
MTRR state to use WB caching as the default.

This is needed in order to cope with the fact that PAT is enabled,
while MTRRs are not supported by the hypervisor.

Fixes: 90b926e68f50 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case")
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20230502120931.20719-5-jgross@suse.com
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>