drm/amdgpu: toggle DF-Cstate to protect DF reg access

driver needs to take DF out Cstate before any DF register
access. otherwise, the DF register may not be accessible.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>