gpu: nvgpu: Begin reorganizing VM mapping/unmapping

Move vm_priv.h to <nvgpu/linux/vm.h> and rename nvgpu_vm_map()
to nvgpu_vm_map_linux(). Also remove a redundant unmap function
from the unmap path. These changes are the beginning of reworking
the nvgpu Linux mapping and unmapping code.

The rest of this patch is just the necessary changes to use the
new map function naming and the new path to the Linux vm header.

Patch Series Goal
-----------------

There's two major goals for this patch series. Note that these
goals are not achieved in this patch. There will be subsequent
patches.

  1.  Remove all last vestiges of Linux code from common/mm/vm.c
  2.  Implement map caching in the common/mm/vm.c code

To accomplish this firstly the VM mapping code needs to have the
struct nvgpu_mapped_buf data struct be completely Linux free. That
means implementing an abstraction for this to hold the Linux stuff
that mapped buffers carry about (SGT, dma_buf). This is why the
vm_priv.h code has been moved: it will need to be included by the
<nvgpu/vm.h> header so that the OS specific struct can be pulled
into struct nvgpu_mapped_buf.

Next renaming the nvgpu_vm_map() to nvgpu_vm_map_linux() is in
preparation for adding a new nvgpu_vm_map() that handles the
map caching with nvgpu_mapped_buf. The mapping code is fairly
straight forward: nvgpu_vm_map does OS generic stuff; each OS
then calls this function from an nvgpu_vm_map_<OS>() or the like
that does any OS specific adjustments/management.

Freeing buffers is much more tricky however. The maps are all
reference counted since userspace does not track buffers and
expects us to handle this instead. Ugh! Since there's ref-counts
the free code will require a callback into the OS specific code
since the OS specific code cannot free a buffer directly. THis
make's the path for freeing a buffer quite convoluted.

JIRA NVGPU-30
JIRA NVGPU-71

Change-Id: I5e0975f60663a0d6cf0a6bd90e099f51e02c2395
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1578896
GVS: Gerrit_Virtual_Submit
Reviewed-by: David Martinez Nieto <dmartineznie@nvidia.com>
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>

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