KVM: arm64: Drop unnecessary masking of PMU registers

We always sanitise our PMU sysreg on the write side, so there
is no need to do it on the read side as well.

Drop the unnecessary masking.

Acked-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210719123902.1493805-3-maz@kernel.org