i915: Bring sanity to the Viewport function

i915: Bring sanity to the Viewport function

The i830 and the i915 driver have the same dd_function_table::Viewport
function... it just has two names and lives in two places.  Using a
single implementation allows cleaning up the saved_viewport nonsense
too.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jljusten@gmail.com>
Cc: Courtney Goeltzenleuchter <courtney@lunarg.com>