drm/amdgpu/pm: Check input value for CUSTOM profile mode setting on legacy SOCs

Check the input value for CUSTOM profile mode setting on legacy
SOCs. Otherwise we may use uninitalized value of input[]

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>