gpu: nvgpu: split the nvgpu_sgt unit from nvgpu_mem

Split the nvgpu_sgt code out from the nvgpu_mem code. Although the
two chunks of code are related the SGT code is distinct and as
such should be its own unit. To do this a new source file has been
added - nvgpu_sgt.c - which contains all the nvgpu_sgt common APIs.
These are the facade APIs to abstract the actual details of how any
given nvgpu_sgt is actually implemented.

An abstract unit - nvgpu_sgt_os - was also defined. This unit
exists solely for the nvgpu_sgt unit to call so that the OS
specific nvgpu_sgt_os_create_from_mem() API can be moved from the
common nvgpu_sgt unit. Note this also updates the name of what the
OS specific units are expected to call. Common code may still use
the generic nvgpu_sgt_create_from_mem() API.

JIRA NVGPU-1391

Change-Id: I37f5b2bbf9f84c0fb6bc296c3e04ea13518bd4d0
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1946012
Reviewed-by: svc-mobile-coverity <svc-mobile-coverity@nvidia.com>
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

userspace/units/mm/page_table/page_table.c

@@ -29,6 +29,7 @@
 #include <nvgpu/gmmu.h>
 #include <nvgpu/mm.h>
 #include <nvgpu/vm.h>
+#include <nvgpu/nvgpu_sgt.h>
 #include <os/posix/os_posix.h>
 
 #include <gk20a/mm_gk20a.h>