gpu: nova-core: vbios: change PmuLookupTableEntry to use size_of

gpu: nova-core: vbios: change PmuLookupTableEntry to use size_of

Annotate the PmuLookupTableEntry with an `#[repr(C, packed)]` attribute.
Removes another magic number by making the struct the same size as the
data it needs to read, allowing the use of
`size_of::<PmuLookupTableEntry>()`

[acourbot@nvidia.com: remove `dead_code` expect to fix `unfulfilled_lint_expectations` lint]

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Rhys Lloyd <krakow20@gmail.com>
Link: https://lore.kernel.org/r/20250718073633.194032-3-krakow20@gmail.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>