drm/amd/amdgpu: disable gfxoff to retrieve gfxclk

For Sienna_Cichlid, GFXOFF state puts gfx dpm into standby mode, then the
gfxclk can't be retireved.

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