drm/amd/pm: fix enum type compared against 0

This less-than-zero comparison of an unsigned value is never true. type < 0U

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>