gpu: nvgpu: zero vidmem pages on allocation

The allocator doesn't give us empty pages, so make sure that they're
full of zeros, just like the sysmem alloc path does.

Jira DNVGPU-16

Change-Id: I0ff8a0718829b13973535ba1111a8a11b91be04d
Signed-off-by: Konsta Holtta <kholtta@nvidia.com>
Reviewed-on: http://git-master/r/1178829
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Vijayakumar Subbu <vsubbu@nvidia.com>

drivers/gpu/nvgpu/gk20a/mm_gk20a.c

@@ -2517,6 +2517,8 @@ int gk20a_gmmu_alloc_attr_vid_at(struct gk20a *g, enum dma_attr attr,
 	mem->size = size;
 	mem->aperture = APERTURE_VIDMEM;
 
+	gk20a_memset(g, mem, 0, 0, size);
+
 	gk20a_dbg_fn("done");
 
 	return 0;