drm/i915: Warn about aux msg buffer vs. size mismatch

If we have have a buffer, we should also have a size, and vice versa.
Let's check it both ways instead of just one.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469717704-13020-1-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>