drm/i915: add MISSING_CASE to a few port/aux power domain helpers

drm/i915: add MISSING_CASE to a few port/aux power domain helpers

MISSING_CASE() would have been useful to track down a recent problem in
intel_display_port_aux_power_domain(), so add it there and a few related
helpers. This was also suggested by Ville in his review of the latest
DMC/DC changes, we forgot to address that.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
[Cherry-picked from drm-intel-next-queued b9fec167 (Imre)]
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448643329-18675-5-git-send-email-imre.deak@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>