memory: render_memory_region: factor out fr constant setters

These 4 replicated lines set properties of fr that are constant over
the course of the function. Factor out their repeated setting (and also
guards against them being set multiple times in the loop below).

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
