drm/i915: Duplicate opregion vbt memory

In the case of vbt residing in opregion, we simply remap the region
into the kernel and pass the memory reference. Instead duplicate the
memory to handle a saner cleanup in intel_bios_init.

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240228213235.2495611-6-radhakrishna.sripada@intel.com