dim: remove redundant warning about BUG

checkpatch already covers "WARNING:AVOID_BUG: Avoid crashing the kernel
- try using WARN_ON & recovery code rather than BUG() or BUG_ON()"

Remove the redundant warning in dim.

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