drm/i915: add a helper to free the members of i915_params

Abstract the one user in anticipation of more. Set the dangling pointers
to NULL while at it.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8637d1e5049dc003718772f19d664aeaf9540856.1545920737.git.jani.nikula@intel.com
