drm/i915: Remove dead signal level debugs

If we ever get here with bogus signal levels we've messed
up somewhere earlier. Just use MISSING_CASE().

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210318161015.22070-3-ville.syrjala@linux.intel.com