mesa: Refactor scissor rectangle setting even more

mesa: Refactor scissor rectangle setting even more

Create an internal function that just writes data into the scissor
rectangle.  In future patches this will see more use because we only
want to call dd_function_table::Scissor once after setting all of the
scissor rectangles instead of once per scissor rectangle.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>