drm/amd/powerplay: disable fw dstate when gfxoff is enabled

SMU FW has bug that it would cause hung when both fw dstate and
gfxoff are enabled at the same time.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>