tests/amdgpu: fix for dispatch/draw test

1. skip test if there's no desired ring
2. clear shader buffer
3. update command buffer for gfx9

Signed-off-by: Flora Cui <flora.cui@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>