drm/amd/powerplay: Off by one in vega20_get_smu_msg_index()

The > should be >= so that we don't read one element beyond the end of
the vega20_message_map[] array.

Fixes: 78031c2c

 ("drm/amd/powerplay: implement smu vega20_message_map for vega20")
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>