drm/nouveau/bar/gm20b: Avoid BAR1 teardown during init

Writing the 0x1704 (BUS_BAR1_BLOCK) register causes the GPU to probe the
memory region at the programmed address. The result is an address decode
error in the external memory controller because address 0, which is what
is written to the register, is not designated as accessible to devices.

Avoid triggering DMA from the GPU by removing teardown of the BAR1.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>