drm/amd/powerplay: avoid repeat AVFS enablement/disablement

No need to enable or disable AVFS if it's already in wanted
state.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>