drm/i915: Split out i915_vma.c

As a side product, had to split two other files;
- i915_gem_fence_reg.h
- i915_gem_object.h (only parts that needed immediate untanglement)

I tried to move code in as big chunks as possible, to make review
easier. i915_vma_compare was moved to a header temporarily.

v2:
- Use i915_gem_fence_reg.{c,h}

v3:
- Rebased

v4:
- Fix building when DEBUG_GEM is enabled by reordering a bit.

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478861034-30643-1-git-send-email-joonas.lahtinen@linux.intel.com