Blender Git Loki

Cleanup: Commonize code for checking scene lights/world settings

There were several places attempting to check to see if scene lights
and world were enabled for display. This tries to find a common place
for both of these to reduce duplication.

Honestly, I couldn't find a really good spot for these and settled on
DRW_engine. It's not the best spot since they're not strictly drawing
related, but let's start here.

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D12658