drm/amdgpu: Increase GFX6 graphics ring size.

To ensure it supports 192 IBs per submission, so we can keep a
simplified IB limit in the follow up patch without having to
look at IP or GPU version.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>