drm/amdgpu: NO KIQ usage on nbio hdp flush routine

nbio hdp flush routine are called within atomic context.
Avoid use KIQ when write to the HDP_MEM_COHERENCY_FLUSH_CNTL register
since this register has its own VF copy

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