drm/mediatek: remove cast to pointers passed to kfree

Remove unnecessary casts to pointer types passed to kfree.
Issue detected by coccinelle:
@@
type t1;
expression *e;
@@

-kfree((t1 *)e);
+kfree(e);

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191023111107.9972-1-wambui.karugax@gmail.com