drm/i915/fdi: convert BUG()'s to MISSING_CASE()

These shouldn't happen, but in the off chance they do, we'll want a
warning rather than panic.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b86132ac63f43f79e51eb63f948beccba85bf449.1629906431.git.jani.nikula@intel.com