gpu: nova-core: register: split @io rule into fixed and relative versions

gpu: nova-core: register: split @io rule into fixed and relative versions

We used the same @io rule with different patterns to define both the
fixed and relative I/O accessors. This can be confusing as the matching
rules are very similar.

Since all call sites know which version they want to call, split @io
into @io_fixed and @io_relative to remove any ambiguity.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20250718-nova-regs-v2-13-7b6a762aa1cd@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>