drm/radeon: allow selection of alignment in the sub-allocator

commit 6c4f978b357bc779c703fda1f200e9179623d3e9 upstream.

There are cases where we need more than 4k alignment.  No
functional change with this commit.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>