rust: dma: use NonNull<T> instead of *mut T

rust: dma: use NonNull<T> instead of *mut T

In struct CoherentAllocation, use NonNull<T> instead of a raw *mut T for
the CPU address; the CPU address of a valid CoherentAllocation won't
ever be NULL.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251103190655.2326191-2-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>