gpu: nova-core: use SizeConstants trait for u64 size constants

gpu: nova-core: use SizeConstants trait for u64 size constants

Replace manual usize-to-u64 conversions of SZ_* constants with the
SizeConstants trait's associated constants on u64. With the
SizeConstants trait in scope, u64::SZ_1M replaces usize_as_u64(SZ_1M)
and similar.

This removes several now-unused imports: usize_as_u64, FromSafeCast,
and individual SZ_* type-level constants.

Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Acked-by: Gary Guo <gary@garyguo.net>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Link: https://patch.msgid.link/20260411024118.471294-2-jhubbard@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>