gpu: nvgpu: Prune redundant cache maintenance

gpu: nvgpu: Prune redundant cache maintenance

Remove redundant cache maintenance operations. Instance blocks and
graphics context buffers are uncached, so they do not need any cache
maintenance.

Bug 1421824

Change-Id: Ie0be67bf0be493d9ec9e6f8226f2f9359cba9f54
Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-on: http://git-master/r/406948
(cherry picked from commit 179d6ff3b2845e99d1719a9ba10862f2d3b22080)
Reviewed-on: http://git-master/r/423231
Reviewed-by: Simone Willett <swillett@nvidia.com>
Tested-by: Simone Willett <swillett@nvidia.com>