drm/i915: Move ggtt fence/alignment to i915_gem_tiling.c

drm/i915: Move ggtt fence/alignment to i915_gem_tiling.c

Rename i915_gem_get_ggtt_size() and i915_gem_get_ggtt_alignment() to
i915_gem_fence_size() and i915_gem_fence_alignment() respectively to
better match usage. Similarly move the pair of functions into
i915_gem_tiling.c next to the fence restrictions.

Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170109161613.11881-6-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>