drm/amdgpu: Do not program VM_L2_CNTL under SRIOV

VM_L2_CNTL* should not be programmed on driver unload under SRIOV.
These regs are skipped during SRIOV driver init.

Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Reviewed-by: Vignesh Chander <Vignesh.Chander@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>