drm/amdgpu: use the maximum possible fragment size on Vega/Raven

The fragment size controls only the L1 on Vega/Raven and we now don't
have any extra overhead any more because of larger fragments.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>