drm/amdgpu: use GPU_HDP_FLUSH for sriov

drm/amdgpu: use GPU_HDP_FLUSH for sriov

Currently SRIOV runtime will use kiq to write HDP_MEM_FLUSH_CNTL for
hdp flush. This register need to be write from CPU for nbif to aware,
otherwise it will not work.

Implement amdgpu_kiq_hdp_flush and use kiq to do gpu hdp flush during
sriov runtime.

v2:
- fallback to amdgpu_asic_flush_hdp when amdgpu_kiq_hdp_flush failed
- add function amdgpu_mes_hdp_flush

v3:
- changed returned error

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Victor Zhao <Victor.Zhao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>