drm/i915: Move i915_vma_move_to_active() to i915_vma.c

i915_vma_move_to_active() has grown beyond its execbuf origins, and
should take its rightful place in i915_vma.c as a method for i915_vma!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706103947.15919-4-chris@chris-wilson.co.uk