x86/bugs: Relocate mds/taa/mmio/rfds defines

Move the mds, taa, mmio, and rfds mitigation enums earlier in the file to
prepare for restructuring of these mitigations as they are all inter-related.

No functional change.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250108202515.385902-3-david.kaplan@amd.com