drm/amd/powerplay: fix the coverity warning about negative check for an unsigned value

There will be a coverity warning because min and max are both unsigned.

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