gallium: add common uploaders into pipe_context (v2)

gallium: add common uploaders into pipe_context (v2)

For lower memory usage and more efficient updates of the buffer residency
list. (e.g. if drivers keep seeing the same buffer for many consecutive
"add" calls, the calls can be turned into no-ops trivially)

v2: add const_uploader, add documentation

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Tested-by: Edmondo Tommasina <edmondo.tommasina@gmail.com>
Tested-by: Charmaine Lee <charmainel@vmware.com>