Variable hub_inst is unused.

Fixes: e28604d8 ("drm/amdkfd: Drop poison hanlding from gfx v10")
Signed-off-by: Jesse Zhang <jesse.zhang@amd.com>
Reviewed-by: Tim Huang <tim.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>