KVM: arm64: Use the reg_to_encoding() macro instead of sys_reg()

The reg_to_encoding() macro is a wrapper over sys_reg() and conveniently
takes a sys_reg_desc or a sys_reg_params argument and returns the 32 bit
register encoding. Use it instead of calling sys_reg() directly.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210106144218.110665-1-alexandru.elisei@arm.com