[ARM] arm64: Minor improvements to VFP save/restore

Currently, VFP save/restore were directly checking CPTR_EL2 value
programmed in HW. This means arch_vcpu_switch() will have to restore
CPTR_EL2 value before doing VFP restore which can lead to bugs
in-future. This patch updates VCPU save/restore to check CPTR_EL2
value from private context instead of HW.

Signed-off-by: Anup Patel <anup@brainfault.org>