virgl: add virgl_shader_binding_state

virgl: add virgl_shader_binding_state

virgl_shader_binding_state will be used to manage all per-stage
shader bindings.  For now, it manages only sampler views.

This replaces virgl_textures_info and fixes some issues

 - start_slot is now honored
 - views outside of [start_slot, slart_slot+count) are unmodified
 - views are released when the context is destroyed

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>