mesa: Separate transform feedback object initialization from allocation.

mesa: Separate transform feedback object initialization from allocation.

Both Gallium and i965 subclass gl_transform_feedback_object, which
requires implementing a custom NewTransformFeedback hook.  Creating a
helper function to initialize the fields avoids code duplication and
divergence.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>