drm/amdgpu/userq: add force completion helpers

Add support for forcing completion of userq fences.
This is needed for userq resets and asic resets so that we
can set the error on the fence and force completion.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>