pan/mdg: Factor out mir_adjust_constant

Each source is semi-independent, we don't need the extra indentation
when the logic is already so complex.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5151>
1 file changed