drm/amdgpu: Fix static checker warning

drivers/gpu/drm/amd/amdgpu/amdgpu_csa.c:49 amdgpu_allocate_static_csa()
error: uninitialized symbol 'ptr'.

the test if (!bo) doesn't work, as the bo is a pointer to a pointer.
if bo create failed, the *bo will be set to NULL.
so change to test *bo.

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