drm/i915: Clean up Makefile

Sorting became confused and a few new files ended up in strange
places. Also move i915_irq.c to core since with the recent-ish
extraction of i915_gpu_error.c and intel_hotplug.c it's more and more
really just basic irq handling code.

When adding new files please don't put them somewhere randomly.

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>