drm/i915: make g4x_digital_port_connected return boolean status

We should not be hitting any of the default cases in
g4x_digital_port_connected, so add MISSING_CASE annotation and return
boolean status. The current behaviour is just cargo culting from the
days of yonder when the display port support was added to i915.

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>