gpu: nvgpu: Increase VA space to 40 bits

gpu: nvgpu: Increase VA space to 40 bits

Now that the buddy allocator is merged we can increase the VA space
without dramatically increasing memory usage by the allocator.

40 bits is the max VA space available on gk20a and gm20b.

Change-Id: I7bc8d86e35b28f041e9a435f2571c8288970c8ee
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: http://git-master/r/745076
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>