mesa: Refactor depth range setting even more

mesa: Refactor depth range setting even more

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

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