drm/radeon/kms: fix 2D tile height alignment in the r600 CS checker

macro tile heights are aligned to num channels, not num banks.

Noticed by Dave Airlie.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>