drm/amd/powerplay: Fix an off by one in navi10_get_smu_msg_index()

The navi10_message_map[] array has SMU_MSG_MAX_COUNT elements so the ">"
has to be changed to ">=" to prevent reading one element beyond the end
of the array.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>