drm/i915/params: don't expose inject_probe_failure in debugfs

The parameter only makes sense as a module parameter only.

Fixes: c43c5a88

 ("drm/i915/params: add i915 parameters to debugfs")
Cc: Juha-Pekka Heikkilä <juha-pekka.heikkila@intel.com>
Cc: Venkata Sandeep Dhanalakota <venkata.s.dhanalakota@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200601215510.18379-1-jani.nikula@intel.com