drm/amd/powerplay: fix the issue of checking on message mapping

The vega20_message_map[index] scope should be in PPSMC_Message_Count not in
SMU_MSG_MAX_COUNT.

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