drm/amdgpu: set snoop bit in pde/pte entries for A+A

Page tables in vram mapping to cpu is changed from uncached to
cached in A+A, the snoop bit in VM_CONTEXTx_PAGE_TABLE_BASE_ADDR/
PDE0s/PDE1s/PDE2s/PTE.TFs has to be set so gpuvm walker snoop
page table data out of CPU cache.

Signed-off-by: Eric Huang <jinhuieric.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>