drm/i915/dsi: Remove weird has_pch_encoder asserts

No idea why the DSI code is feeling the need to assert that
has_pch_encoder must not be set. PCH encoders aren't even a
thing on any platform that has DSI.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230608203057.23759-14-ville.syrjala@linux.intel.com