main: Added entry point for glCreateTransformFeedbacks

main: Added entry point for glCreateTransformFeedbacks

v2: Review from Laura Ekstrand
- generate the name of the gl method once
- shorten some lines to stay in the 78 chars limit

v3: Review from Fredrik Höglund <fredrik@kde.org>
- rename gl_mthd_name to func
- set EverBound in _mesa_create_transform_feedbacks in the dsa case

v4:
- rename _mesa_create_transform_feedbacks to create_transform_feedbacks and
  make it static

Reviewed-by: Laura Ekstrand <laura@jlekstrand.net>
Signed-off-by: Martin Peres <martin.peres@linux.intel.com>