KVM: arm64: vgic-v3: Convert userspace accessors over to FIELD_GET/FIELD_PREP

The GICv3 userspace accessors are all about dealing with conversion
between fields from architectural registers and internal representations.

However, and owing to the age of this code, the accessors use
a combination of shift/mask that is hard to read. It is nonetheless
easy to make it better by using the FIELD_{GET,PREP} macros that solely
rely on a mask.

This results in somewhat nicer looking code, and is probably easier
to maintain.

Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>