drm/i915/ddi: warn instead of oops on invalid ddi encoder type

It's more useful to limp on than bring the kernel down. Hitting this is
a more likely event with BXT DSI, although care should be taken not to
call the function for DSI.

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