drm/amdgpu: Enter low power state if CRTC active.

CRTC in DPMS state off calls for low power state entry.
Support both atomic mode setting and pre-atomic mode setting.

v2: move comment

Acked-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>