KVM: x86/mmu: Use KVM_PAGES_PER_HPAGE() instead of an open coded equivalent

Use KVM_PAGES_PER_HPAGE() instead of open coding equivalent logic that is
anything but obvious.

No functional change intended, and verified by compiling with the below
assertions:

        BUILD_BUG_ON((1UL << KVM_HPAGE_GFN_SHIFT(PG_LEVEL_4K)) !=
                      KVM_PAGES_PER_HPAGE(PG_LEVEL_4K));

        BUILD_BUG_ON((1UL << KVM_HPAGE_GFN_SHIFT(PG_LEVEL_2M)) !=
                      KVM_PAGES_PER_HPAGE(PG_LEVEL_2M));

        BUILD_BUG_ON((1UL << KVM_HPAGE_GFN_SHIFT(PG_LEVEL_1G)) !=
                      KVM_PAGES_PER_HPAGE(PG_LEVEL_1G));

Link: https://lore.kernel.org/r/20240809194335.1726916-19-seanjc@google.com


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