drm/i915/display: make struct __intel_global_objs_state opaque

With struct __intel_global_objs_state only being accessed in
intel_global_state.c, we can make it opaque. The double underscore to
indicate internal becomes redundant, drop it.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/06cc4d1c506e3a5b1cc50e01c4bd1135bbf0f7bd.1753953530.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>