KVM: x86/mmu: Refactor is_tdp_mmu_root into is_tdp_mmu

mainline inclusion
from mainline-v5.14-rc1
commit 63c0cac9
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I7S3VQ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=63c0cac938edfa5d72bfbe8f1eeb9d47b397829c



----------------------------------------------------------------------

This change simplifies the call sites slightly and also abstracts away
the implementation detail of looking at root_hpa as the mechanism for
determining if the mmu is the TDP MMU.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20210617231948.2591431-4-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>