drm/i915: Split i915_gem_batch_pool into its own header

In the next patch, I want to use the structure elsewhere and so require
it defined earlier. Rather than move the definition to an earlier location
where it feels very odd, place it in its own header file.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>