i965: Generalize the update_null_renderbuffer_surface vtbl hook to non-renderbuffers.

i965: Generalize the update_null_renderbuffer_surface vtbl hook to non-renderbuffers.

Null surfaces are going to be useful to have something to point
unbound image units to, as the ARB_shader_image_load_store extension
requires us to behave deterministically in cases where some shader
tries to access an unbound image unit: Invalid stores and atomics are
supposed to be discarded and invalid loads are supposed to return
zero, which is precisely what the null surface does.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>