drm/amdkfd: Use gpu_offset for user queue's wptr

Directly use tbo's start address will miss the domain start offset. Need
to use gpu_offset instead.

Signed-off-by: YuBiao Wang <YuBiao.Wang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org