drm/amdgpu: revert "use more than 64KB fragment size if possible"

This reverts commit 1dcd32fb9c54334ec948a0f18174a748d6b14364.

The block size is indeed an equal match, so this can cause performance regressions.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org