drm/amd/powerplay: fix static checker warnings in iceland_smc.c

add array length check to avoid buffer overflow.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>