drm/i915: [dinq] shut up six instances of -Warray-bounds

Introduced in commits c1cd90ed and d27b1e0e

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: s/fix/shut up in the commit msg and add a comment to the
BUG_ON.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>