MFC r349645:
Remove dead code added after r348743 in the LinuxKPI. The
LINUXKPI_VERSION macro is not defined for any compiled LinuxKPI code
which basically means __GFP_NOTWIRED is never checked when allocating
pages. This should work fine with the existing external DRM code as
long as the page wiring and unwiring is balanced.

Sponsored by: Mellanox Technologies