st/mesa: add an entirely separate codepath for setting up buffer views

st/mesa: add an entirely separate codepath for setting up buffer views

Remove handling of buffers from all texture paths.
This simplifies things for both buffers and textures.

get_sampler_view_format is also cleaned up not to call
util_format_is_depth_and_stencil.

v2: also update st_NewTextureHandle

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com> (v1)