drm/radeon/kms: fix alignment when allocating buffers

We were previously dropping alignment requests on the floor
when allocating buffers so we always ended up page aligned.
Certain tiling modes on 6xx+ require larger alignment which
wasn't happening before.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: Jerome Glisse <j.glisse@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>