drm/i915: Use BUG() in a case of a programming error

The port number should always be correctly set. Do the same thing as the
switch above and use BUG() to signal that branch is not supposed to be
taken.

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