drm/i915: inline enable/disable_irq into ring->get/put_irq

Now that these are properly refactored this additional indirection
doesn't really buy us anything but confusion. Hence inline them.

This duplicates the ironlake gt enable/disable code snippet, but we've
already separate ilk from gen6+ gt irq in i915_irq.c, so I think this
makes more sense.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>