gpu: nova-core: use CStr::from_bytes_until_nul() and remove util.rs

gpu: nova-core: use CStr::from_bytes_until_nul() and remove util.rs

The util.rs module contained a single helper function,
str_from_null_terminated(), which duplicated functionality that is now
available in core::ffi::CStr.

Specifically, CStr::from_bytes_until_nul() is available in the kernel's
minimum supported Rust version (1.78.0), so it time to stop using this
custom workaround.

Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Link: https://patch.msgid.link/20260106035226.48853-2-jhubbard@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>