Add new extension to support setting an immutable-priority-list of
potential placements, at creation time.

If we use the normal gem_create or gem_create_ext without the
extensions/placements then we still get the old behaviour with only
placing the object in system memory.

v2(Daniel & Jason):
    - Add a bunch of kernel-doc
    - Simplify design for placements extension

Testcase: igt/gem_create/create-ext-placement-sanity-check
Testcase: igt/gem_create/create-ext-placement-each
Testcase: igt/gem_create/create-ext-placement-all
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: CQ Tang <cq.tang@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@linux.intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Dave Airlie <airlied@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: mesa-dev@lists.freedesktop.org
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210429103056.407067-6-matthew.auld@intel.com