drm/amdgpu/userq: unpin and unref doorbell and wptr outside mutex

drm/amdgpu/userq: unpin and unref doorbell and wptr outside mutex

In amdgpu_userq_destroy once unmap_helpder is called within mutex
there is no need to hold mutex.

This helps in avoiding a deadlock between doorbell and wptr ww mutex
and we could unpin and unref these bos outside mutex safely.

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>