drm/amd/powerplay: functions's return state was reversed

In commit 195567e9,
use true/false instead of 1/0 to fix build warning.
But the original logic: '0' means true and '1' means false.

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