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

We align to 64KB, but when userspace aligns even more we can easily use more.

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