mesa: signal driver when buffer is bound to different texture format

Gallium caches sampler states for TBOs. Now if a buffer is first attached to a TBO specifying one format, and later attached by specifying another format. When this TBO is then used, that would lead to an assertion failure in debug builds, or to invalid rendering in release builds, because the TBO picks the original, wrong format for the sampler view.

Resolve this by signalling the change to Gallium (and other drivers), so that Gallium clears the sampler view cache.