drm/amdgpu: To get gds, gws and oa from adev->gds (v2)

As now enabled per vm bo feature, the user mode driver won't supply the
bo_list generally, for this case, the gdb_base, gds_size, gws_base, gws_size and
oa_base, oa_size won't be set.

v2: fix warning (Chunming)

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>