KVM: x86/mmu: Use KVM-governed feature framework to track "GBPAGES enabled"

Use the governed feature framework to track whether or not the guest can
use 1GiB pages, and drop the one-off helper that wraps the surprisingly
non-trivial logic surrounding 1GiB page usage in the guest.

No functional change intended.

Reviewed-by: Yuan Yao <yuan.yao@intel.com>
Link: https://lore.kernel.org/r/20230815203653.519297-3-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>