drm/amdkfd: Remove unnecessary cast in kfree

Remove an unnecassary cast in the argument to kfree.

Found using Coccinelle. The semantic patch used to find this is as follows:

//<smpl>
@@
type T;
expression *f;
@@

- kfree((T *)(f));
+ kfree(f);
//</smpl>

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>