drm/amdgpu: switch job hw_fence to amdgpu_fence

Use the amdgpu fence container so we can store additional
data in the fence.  This also fixes the start_time handling
for MCBP since we were casting the fence to an amdgpu_fence
and it wasn't.

Fixes: 3f4c175d ("drm/amdgpu: MCBP based on DRM scheduler (v9)")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>