drm/amd/powerplay: using valid mapping check for rn

Check whether the message mapping is valid

Signed-off-by: Aaron Liu <aaron.liu@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>