drm/i915: BUG() on unexpected HDMI register

This should never happen, but the silent "return" makes me wonder
every time I try to debug InfoFrame bugs, so promote this to BUG() to
make sure people will complain if we ever break this.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>