drm/amd/powerplay: fix the incorrect return value

The return value 0 (false) means fail to find GPIO in
atomctrl_get_pp_assign_pin. "-1" returns true as bool actually.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>