drm_global.c: Destroy sx in drm_global_release()
drm_global.c: Destroy sx in drm_global_release()

This fixes a build error at the same time (unused variable "item"), if
the kernel is compiled without INVARIANTS.

Reported by:	Hartmann, O. <ohartman@zedat.fu-berlin.de> (build error)
Reviewed by:	Konstantin Belousov (kib@)

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