drm/i915: Make i915_vma_destroy() static

i915_vma_destroy() is now not used outside of i915_vma.c so we can
remove the export and make the function static.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170616123508.12673-1-chris@chris-wilson.co.uk


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>