KVM: arm64: Drop raz parameter from read_id_reg()

There is no longer a need for caller-specified RAZ visibility. Hoist the
call to sysreg_visible_as_raz() into read_id_reg() and drop the
parameter.

No functional change intended.

Suggested-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220913094441.3957645-4-oliver.upton@linux.dev