drm: Change the return type of the unload hook to void

The integer returned by the unload hook is ignored by the drm core, so
let's make it void.

This patch was created using the following Coccinelle semantic script
(except for the declaration and comment in drm_drv.h):

Compile-tested only.

// <smpl>
@ get_name @
struct drm_driver drv;
identifier fn;
@@
drv.unload = fn;

@ replace_type @
identifier get_name.fn;
@@
- int
+ void
fn (...)
{
...
}

@ remove_return_param @
identifier get_name.fn;
@@
void fn (...)
{
<...
if (...)
return
- ...
;
...>
 }

@ drop_final_return @
identifier get_name.fn;
@@
void fn (...)
{
...

- return 0;
}
// </smpl>

Suggested-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Acked-by: Christian König <christian.koenig@amd.com&gt;.>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170106175731.29196-1-krisman@collabora.co.uk