Make staging driver stable for SRIOV VF (1)

[Pixel Ding]

This is the first patch series to make latest staging driver
stable for SRIOV VF on both Tonga and Vega. Patches are merged
from SRIOV branches or reimplemented, including bug fixes and
small features requested by SRIOV users.  

Please help reviewing, Thanks.

[PATCH 1/4] drm/amdgpu: always consider virualised device for
[PATCH 2/4] drm/amdgpu: workaround for VM fault caused by SDMA
[PATCH 3/4] drm/amdgpu: report preemption fence via amdgpu_fence_info
[PATCH 4/4] drm/amdgpu: busywait KIQ register accessing