mesa: compute _PipeFormat and _ElementSize after checking if the format changed

Split _mesa_set_vertex_format into 2 functions and set the derived fields
after we know that the format is different.

This skips _PipeFormat and _ElementSize recomputation every time we get
gl*Pointer with the same format.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20287>