drm/amdgpu: skip unload tmr when tmr is not loaded

[why]
Skip TMR unload for Navi12 and CHIP_SIENNA_CICHLID SRIOV as TMR is
not loaded at all

Signed-off-by: Tong Liu01 <Tong.Liu01@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>