drm/i915: Remove dead TPS3->TPS2 fallback code

If we ever get here with TPS3 then intel_dp_training_pattern()
is just broken. Replace the careful fallback with just
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/20210318161015.22070-2-ville.syrjala@linux.intel.com