optimize entity_get_render_display() for the case of "multiple entity view"

Right now EntityRenderController::viewMultiple() calls entity_get_render_display() once per bundle, resulting in separate entity loads.
We should optimize this, collect all involved bundles and then load the corresponding displays in a single load.

This might mean moving entity_get_render_display() to a method in EntityRenderController, which would only make sense IMO. This is "determine the display object to use when rendering an entity".