drm/amdgpu/userq: Call unreserve on error in amdgpu_userq_fence_read_wptr()

This error path should call amdgpu_bo_unreserve() before returning.

Fixes: d8675102 ("drm/amdgpu: add vm root BO lock before accessing the vm")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>