drm/ast: Fix HW cursor image

commit 1e99cfa8de0f0879091e33cd65fd60418d006ad9 upstream.

The translation from the X driver to the KMS one typo'ed a couple
of array indices, causing the HW cursor to look weird (blocky with
leaking edge colors). This fixes it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>