KVM: arm64: Helper to write to appropriate feature trap register based on mode

Factor out the code that decides whether to write to the feature
trap registers, CPTR_EL2 or CPACR_EL1, based on the KVM mode,
i.e., (h)VHE or nVHE.

This function will be used in the subsequent patch.

No functional change intended.

Signed-off-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230724123829.2929609-6-tabba@google.com


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>