drm/amdgpu: add dummy event6 for vega10

[why]
Malicious mailbox event1 fails driver loading on vega10.
A dummy event6 prevent driver from taking response from malicious event1 as its own.

[how]
On vega10, send a mailbox event6 before sending event1.

Signed-off-by: James Yao <yiqing.yao@amd.com>
Reviewed-by: Jingwen Chen <Jingwen.Chen2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>