drm/amdgpu:no kiq for mailbox registers access

Use no kiq version reg access due to:
1) better performance
2) INTR context consideration (some routine in mailbox is in
   INTR context e.g.xgpu_vi_mailbox_rcv_irq)

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>