drm/amdgpu: use 2MB fragment size for GFX6,7 and 8

Use 2MB fragment size by default for older hardware generations as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: John Bridgman <john.bridgman@amd.com>
Reviewed-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>