drm/amdgpu/userq: move wptr_obj cleanup in mqd_destroy

drm/amdgpu/userq: move wptr_obj cleanup in mqd_destroy

In case when queue_create fails and mqd has already been
allocated and hence wptr_obj is not cleaned up.

So moving that cleanup part to mqd_destroy so it takes
care of all the cases of clean up and during tear down of
the queue.

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 43355f62cd2ef5386c2693df537c232ea0f2ce6c)