drm/amdgpu: Set power ungate state when suspend/fini

Unify to set power ungate state at the begin of suspend/fini.
Remove the workaround code for gfx off feature in
amdgpu_device.c.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>