ANDROID: KVM: arm64: Check for PTE valitity when checking for executable/cacheable

Don't blindly assume that the PTE is valid when checking whether
it describes an executable or cacheable mapping.

This makes sure that we don't issue CMOs for invalid mappings.

Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Bug: 209580772
Change-Id: I5b271c91aa6ceb23f7b1e6a571e30d080866d5c9
Signed-off-by: Will Deacon <willdeacon@google.com>