drm/amd/powerplay: incorrectly use of the function return value

'0' means true.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org