drm/ast: Fix HW cursor image

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>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>