gallium: change pipe_sampler_view::first_element/last_element -> offset/size

gallium: change pipe_sampler_view::first_element/last_element -> offset/size

This is required by OpenGL. Our hardware supports this.

Example: Bind RGBA32F with offset = 4 bytes.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97305

Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Nicolai Hähnle <nicolai.haehnle@amd.com>