drm/amdgpu: jt_size was wrongly counted twice

Signed-off-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>