drm amdgpu: Skip tmr load for SRIOV

1. For Navi12, CHIP_SIENNA_CICHLID, skip tmr load operation;
2. Check pointer before release firmware.

v2: use CHIP_SIENNA_CICHLID instead
v3: remove local "bool ret"; fix grammer issue
v4: use my name instead of "root"
v5: fix grammer issue and indent issue

Signed-off-by: Liu ChengZhe <ChengZhe.Liu@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>