drm/amdgpu: change gfx8 ib test to use WB

This patch is extends the usage of WB in
gfx8's ib test which was originally
implemented in the below upstream patch
"ed9324af drm/amdgpu: change gfx9 ib test to use WB"

For reference below are the reasons for switching
to WB:

1)Because when doing IB test we don't want to involve KIQ health
status affect, and since SCRATCH register access is go through
KIQ that way GFX IB test would failed due to KIQ fail.

2)acccessing SCRATCH register cost much more time than WB method
because SCRATCH register access runs through KIQ which at least could
begin after GPU world switch back to current Guest VF

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>