drm/radeon/kms: fix typo in atom overscan setup

commit 942b0e95 upstream.

Typo in the aspect scale setup.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>