drm/i915/opregion: abstract getting the opregion VBT

Add a function to get the opregion VBT instead of accessing the opregion
structures directly.

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/8205b8fa724f98bbf1f76c59e661909d874e843e.1704992868.git.jani.nikula@intel.com