KVM: arm64: Rely on index_to_param() for size checks on userspace access

index_to_param() already checks that we use 64bit accesses for all
registers accessed from userspace.

However, we have extra checks in other places (such as index_to_params),
which is pretty confusing. Get rid off these redundant checks.

Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>