drm/amd/pm: check negtive return for table entries

Function hwmgr->hwmgr_func->get_num_of_pp_table_entries(hwmgr) returns a negative number

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