drm/i915: remove unused orig_clock i915 member

Unused since commit f97108d1 ("drm/i915: add dynamic performance
control support for Ironlake"). That's a little over ten years. Good
riddance.

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/20200227170047.31089-2-jani.nikula@intel.com