drm/amd/powerplay: avoid enabling/disabling uvd/vce dpm twice

For vega20, there are two UVD rings which share one powerplay instance.
Under some case(two rings used parallel), the uvd dpm is disabled twice
which causes the SMC hang.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Rex Zhu <rezhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>