st/nine: Check if format is DS before retrieving flags

st/nine: Check if format is DS before retrieving flags

d3d9_get_pipe_depth_format_bindings assumes the input format
is a depth stencil format.
Previously the user could hit this function with an invalid format.
Protect the last non protected call with a depth_stencil_format check.

Another solution is to have d3d9_get_pipe_depth_format_bindings
support non depth stencil format, but we don't want the user
to create depth buffers with d3d formats that can't be one,
it's better to check if the format can be depth buffer with d3d.

Signed-off-by: Axel Davy <davyaxel0@gmail.com>