drm/i915/params: prevent changing module params runtime

Only support runtime changes through the debugfs.

i915.verbose_state_checks remains an exception, and is not exposed via
debugfs.

This depends on IGT having been updated to use the debugfs for modifying
the parameters.

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/20200601215510.18379-3-jani.nikula@intel.com