drm/i915/display: stop using BUG()

Avoid bringing the entire machine down even if there's a bug that
shouldn't happen, but won't corrupt the system either. Log them loudly
and limp on.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220531162527.1062319-1-jani.nikula@intel.com