nvk: Add support for contiguous heaps to nvk_heap

nvk: Add support for contiguous heaps to nvk_heap

This is required for shader heaps pre-Volta because they use a single
64-bit base address and 32-bit offsets for individual shaders.  In this
case, the addresses returned from the heap are relative to the base.
The BO for the heap can be retrieved via nvk_heap_get_contiguous_bo_ref
which returns an actual reference so it's safe to call from the queue
submit path.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24326>