st/nine: Fix check and remove useless code in swapchain9

st/nine: Fix check and remove useless code in swapchain9

The removed code was there for two reasons:
1) Allow DF16, DF24, INTZ to be used as depth buffer
for swapchain, if the driver doesn't support
PIPE_BIND_SAMPLER_VIEW for the underlying format
2) Set PIPE_BIND_SAMPLER_VIEW if possible, such that
if StretchRect is called on the depth texture, it is happy.

1) The reason these formats needed a workaround is because
the check flags for them in CheckDeviceFormat were incorrect,
which led applications to think the formats were valid for
swapchains, even if they weren't supported.
2) StretchRect limitations for depth buffers force
the resource_copy_region path, which should be fine without
PIPE_BIND_SAMPLER_VIEW.

Thus fix the check for 1), and remove the code.

Signed-off-by: Axel Davy <axel.davy@ens.fr>