gpu: nvgpu: Allow mapping backing store

Backing store sgt needs to be mapped to gpuva to enable CDE swizzling.
This patch adds necessary code to create sgt during initialisation so
that the sgt is available when needed.

Bug 1409151

Change-Id: I9d4671386fe9204d780c2e286b5f9b2dd87af35a
Signed-off-by: Arto Merilainen <amerilainen@nvidia.com>

drivers/gpu/nvgpu/gk20a/mm_gk20a.h

@@ -157,6 +157,7 @@ struct gr_ctx_desc {
 
 struct compbit_store_desc {
 	struct page **pages;
+	struct sg_table *sgt;
 	size_t size;
 	u64 base_iova;
 };