gallium: clarify the constraints on sampler_view_destroy

gallium: clarify the constraints on sampler_view_destroy

r600 expects the context that created the sampler view to still be alive
(there is a per-context list of sampler views).

svga currently bails when the context of destruction is not the same as
creation.

The GL state tracker, which is the only one that runs into the
multi-context subtleties (due to share groups), already guarantees that
sampler views are destroyed before their context of creation is destroyed.

Most drivers are context-agnostic, so the warning message in
pipe_sampler_view_release doesn't really make sense.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>