drm/amdgpu: Put enable gfx off feature to a delay thread

delay to enable gfx off feature to avoid gfx on/off frequently
suggested by Alex and Evan.

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