linuxkpi: Support non-NULL zero-size pointers

DRM drivers set some pointers to ZERO_SIZE_PTR directly (without
allocating anything), to treat pointers which were "initialized" (set to
ZERO_SIZE_PTR) with no memory allocation like really allocated
pointers. NULL isn't used because it represents a third state.

Reviewed by: emaste, manu
Approved by: emaste, manu
Differential Revision: https://reviews.freebsd.org/D39055