drm/i915: add MISSING_CASE annotation to ibx_digital_port_connected

With the case added for eDP on port A (always connected from this
function's point of view), we should not be hitting any of the default
cases in ibx_digital_port_connected, so add MISSING_CASE annotation.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>