drm/i915/opregion: make struct intel_opregion opaque

With the recent cleanups, only intel_opregion.c needs to know the
definition of struct intel_opregion. Allocate it dynamically and make it
opaque.

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/3b68d7ff4b2930eaf15d9657618a738b9065f64b.1704992868.git.jani.nikula@intel.com