drm/amdgpu: move static CSA address to top of address space v2

Move the CSA area to the top of the VA space to avoid clashing with
HMM/ATC in the lower range on GFX9.

v2: wrong sign noticed by Roger, rebase on CSA_VADDR cleanup, handle VA
hole on GFX9 as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>