anv/pipeline: Fix up deref modes if we delete a FS output

anv/pipeline: Fix up deref modes if we delete a FS output

With the new deref instructions, we have to keep the modes consistent
between the derefs and the variables they reference.  Since we remove
outputs by changing them to local variables, we need to run the fixup
pass to fix the modes.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>