drm/i915: dmesg output for VT-d testing

Our validation guys want to have a positive proof that the gfx driver
is indeed using VT-d, since setting up a gfx stack, especially in
early bring-up and by people not versed in linux gfx is a bit tricky.
So provide just that.

Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>