gpu: nvgpu: Fix pgsz_idx used in gk20a_vm_alloc_space()

gpu: nvgpu: Fix pgsz_idx used in gk20a_vm_alloc_space()

Use the correct page size index for pgsz_idx in gk20a_vm_alloc_space().
Previously the page size itself was used, not the page size index.

Bug 1837624

Change-Id: I652f5af5321c1c49dc8eb170d3f92f00c23d2b6f
Signed-off-by: Alex Waterman <alexw@nvidia.com>
(cherry picked from commit fd13e0e1c4e397335c24497a0f92c85934d6185f)
Reviewed-on: http://git-master/r/1503371
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-by: Winnie Hsu <whsu@nvidia.com>