drm/i915/opregion: move i915_opregion debugfs to intel_opregion.c

All things about opregion should be placed in intel_opregion.c.

Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c10103d2dd775edc1e9f93f09d0834480f880596.1704992868.git.jani.nikula@intel.com