nv50-: separate vertex formats from surface format descriptions

nv50-: separate vertex formats from surface format descriptions

We've previously had identical naming between vertex and texture
formats, so it mostly made sense to define these together.

However, upcoming patches are going to transition the driver over to
using updated texture header definitions using NVIDIA's naming, and this
will no longer be the case.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>