gallium: another interface change for multisampling

gallium: another interface change for multisampling

due to popular request, use nr_samples parameter in is_format_supported()
instead of new is_msaa_supported() query.
This makes it easily possible to query if a format with a given sample count
is also supported not only as render target, but for sampler views (note that
texture sampling from multisampled resources isn't supported yet).
It is not quite how dx10 format msaa queries work, but we might need to revisit
format queries completely in the future anyway.