drm/i915: add a helper to make a copy of i915_params

Abstract the one user in anticipation of more.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c6a94b4da8dc723df025b1f602fe46d76d00d53f.1545920737.git.jani.nikula@intel.com