arm64/sve: Factor out FPSIMD to SVE state conversion

Currently we convert from FPSIMD to SVE register state in memory in
two places.

To ease future maintenance, let's consolidate this in one place.

Reviewed-by: Julien Grall <julien.grall@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>