gpu: nvgpu: Refactor page mapping code

gpu: nvgpu: Refactor page mapping code

Pass always the directory structure to mm functions instead of
pointers to members to it. Also split update_gmmu_ptes_locked()
into smaller functions, and turn the hard
coded MMU levels (PDE, PTE) into run-time parameters.

Change-Id: I315ef7aebbea1e61156705361f2e2a63b5fb7bf1
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-on: http://git-master/r/672485
Reviewed-by: Automatic_Commit_Validation_User