drm/amdgpu: Ctrl gfx off via amdgpu_gfx_off_ctrl

use amdgpu_gfx_off_ctrl function so driver can arbitrate
whether the gfx ip can be power off or power on.

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