eventdev: introduce helper function for enqueue burst

Introducing a helper function to avoid duplicating
common enqueue burst code when introducing
enqueue burst variants.

Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Gage Eads <gage.eads@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>