gallium: add an interface for memory allocations.

gallium: add an interface for memory allocations.

In order to support vulkan over gallium for the sw renderers,
there needs to be a vulkan-like memory allocation API.

It doesn't need to be overly complicated for the needs of the sw
renderers.

The vallium layer will allocate resources and memory separately
and bind them via this API.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6082>