drm/i915: Takedown drm_mm on failed gtt setup

This was found by code inspection. If the GTT setup fails then we are
left without properly tearing down the drm_mm.

Hopefully this never happens.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>