gpu: nova-core: add FbRange.len() and use it in boot.rs

gpu: nova-core: add FbRange.len() and use it in boot.rs

A tiny simplification: now that FbLayout uses its own specific FbRange
type, add an FbRange.len() method, and use that to (very slightly)
simplify the calculation of Frts::frts_size initialization.

Suggested-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Link: https://patch.msgid.link/20260310021125.117855-3-jhubbard@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>