drm/i915: Fixup kerneldoc includes

Would be great if everony could add

$ make DOCBOOKS="" htmldocs

to their build scripts to catch these. 0day should also report them,
not sure why it failed to spot this.

Fixes: b42fe9ca ("drm/i915: Split out i915_vma.c")
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114115825.22050-3-daniel.vetter@ffwll.ch