rust: drm: remove unnecessary imports

`kernel::str::CStr` is included in the prelude.

Signed-off-by: Tamir Duberstein <[e-mail]>
Signed-off-by: Danilo Krummrich <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/drm_panic_qr.rs

···
 //! * <https://github.com/erwanvivien/fast_qr>
 //! * <https://github.com/bjguillot/qr>
 
-use kernel::{prelude::*, str::CStr};
+use kernel::prelude::*;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Ord, PartialOrd)]
 struct Version(usize);

rust/kernel/drm/driver.rs

···
     drm,
     error::{to_result, Result},
     prelude::*,
-    str::CStr,
     types::ARef,
 };
 use macros::vtable;