drm/panic: use `core::ffi::CStr` method names

Prepare for `core::ffi::CStr` taking the place of `kernel::str::CStr` by
avoid methods that only exist on the latter.

Link: https://github.com/Rust-for-Linux/linux/issues/1075
Acked-by: Greg Kroah-Hartman <[e-mail]>
Reviewed-by: Alice Ryhl <[e-mail]>
Reviewed-by: Benno Lossin <[e-mail]>
Acked-by: Danilo Krummrich <[e-mail]>
Signed-off-by: Tamir Duberstein <[e-mail]>
Acked-by: Jocelyn Falempe <[e-mail]>
Signed-off-by: Miguel Ojeda <[e-mail]>

drivers/gpu/drm/drm_panic_qr.rs

···
         // nul-terminated string.
         let url_cstr: &CStr = unsafe { CStr::from_char_ptr(url) };
         let segments = &[
-            &Segment::Binary(url_cstr.as_bytes()),
+            &Segment::Binary(url_cstr.to_bytes()),
             &Segment::Numeric(&data_slice[0..data_len]),
         ];
         match EncodedMsg::new(segments, tmp_slice) {