drm/amd/pm: suppress static checker warning

Suppress the warning below:
drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/hardwaremanager.c:274 phm_check_smc_update_required_for_display_configuration()
warn: signedness bug returning '(-22)'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>