drm/amdgpu/gfx8: Fix compute ring failure after resetting

Do ring clear before ring test, otherwise compute ring test will
fail after gpu resetting. Still can't find the root cause, just
workaround it.

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