drm/i915: ensure correct VLV IOSF SB units have been get/put

Add some extra paranoia to check correct use of the VLV IOSF SB
get/put/read/write.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/be6f029adf7148198821e5fe01dcf5d5406aa1fe.1747061743.git.jani.nikula@intel.com


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